diff --git a/dev/bots/firebase_testlab.sh b/dev/bots/firebase_testlab.sh
index 4d80681145..90f1f9a39f 100755
--- a/dev/bots/firebase_testlab.sh
+++ b/dev/bots/firebase_testlab.sh
@@ -3,6 +3,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+set -e
+
 # The tests to run on Firebase Test Lab.
 # Currently, the test consists on building an Android App Bundle and ensuring
 # that the app doesn't crash upon startup.
@@ -26,8 +28,6 @@ devices=(
   "model=griffin,version=24"
 )
 
-set -e
-
 GIT_REVISION=$(git rev-parse HEAD)
 
 DEVICE_FLAG=""
@@ -35,6 +35,12 @@ for device in ${devices[*]}; do
   DEVICE_FLAG+="--device $device "
 done
 
+# If running tests locally, the key env var needs to be set.
+if [[ -z $GCLOUD_FIREBASE_TESTLAB_KEY ]]; then
+  echo "Not running firebase test lab tests because GCLOUD_FIREBASE_TESTLAB_KEY isn't set."
+  exit 0
+fi
+
 # New contributors will not have permissions to run this test - they won't be
 # able to access the service account information. We should just mark the test
 # as passed - it will run fine on post submit, where it will still catch
diff --git a/packages/flutter/lib/src/services/binary_messenger.dart b/packages/flutter/lib/src/services/binary_messenger.dart
index 7b24275472..4b27557d01 100644
--- a/packages/flutter/lib/src/services/binary_messenger.dart
+++ b/packages/flutter/lib/src/services/binary_messenger.dart
@@ -48,6 +48,9 @@ abstract class BinaryMessenger {
   ///
   /// This method is useful for tests or test harnesses that want to assert the
   /// handler for the specified channel has not been altered by a previous test.
+  ///
+  /// Passing null for the `handler` returns true if the handler for the
+  /// `channel` is not set.
   bool checkMessageHandler(String channel, MessageHandler? handler);
 
   /// Set a mock callback for intercepting messages from the [send] method on
@@ -69,6 +72,9 @@ abstract class BinaryMessenger {
   /// This method is useful for tests or test harnesses that want to assert the
   /// mock handler for the specified channel has not been altered by a previous
   /// test.
+  ///
+  /// Passing null for the `handler` returns true if the handler for the
+  /// `channel` is not set.
   bool checkMockMessageHandler(String channel, MessageHandler? handler);
 }
 
diff --git a/packages/flutter/lib/src/services/platform_channel.dart b/packages/flutter/lib/src/services/platform_channel.dart
index 316af4eaba..6bf0fd367d 100644
--- a/packages/flutter/lib/src/services/platform_channel.dart
+++ b/packages/flutter/lib/src/services/platform_channel.dart
@@ -389,6 +389,9 @@ class MethodChannel {
   ///
   /// This method is useful for tests or test harnesses that want to assert the
   /// handler for the specified channel has not been altered by a previous test.
+  ///
+  /// Passing null for the `handler` returns true if the handler for the channel
+  /// is not set.
   bool checkMethodCallHandler(Future<dynamic> Function(MethodCall call)? handler) => _methodChannelHandlers[this] == handler;
 
   /// Sets a mock callback for intercepting method invocations on this channel.
@@ -422,6 +425,9 @@ class MethodChannel {
   ///
   /// This method is useful for tests or test harnesses that want to assert the
   /// handler for the specified channel has not been altered by a previous test.
+  ///
+  /// Passing null for the `handler` returns true if the handler for the channel
+  /// is not set.
   bool checkMockMethodCallHandler(Future<dynamic> Function(MethodCall call)? handler) => _methodChannelMockHandlers[this] == handler;
 
   Future<ByteData?> _handleAsMethodCall(ByteData? message, Future<dynamic> handler(MethodCall call)) async {
diff --git a/packages/flutter/test/services/asset_bundle_test.dart b/packages/flutter/test/services/asset_bundle_test.dart
index eb014c182f..c3517de2ce 100644
--- a/packages/flutter/test/services/asset_bundle_test.dart
+++ b/packages/flutter/test/services/asset_bundle_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:convert';
 import 'dart:typed_data';
 
@@ -41,7 +39,7 @@ void main() {
 
     expect(bundle.loadCallCount['one'], 1);
 
-    Object loadException;
+    late Object loadException;
     try {
       await bundle.loadString('foo');
     } catch (e) {
@@ -61,7 +59,7 @@ void main() {
   test('NetworkAssetBundle control test', () async {
     final Uri uri = Uri.http('example.org', '/path');
     final NetworkAssetBundle bundle = NetworkAssetBundle(uri);
-    FlutterError error;
+    late FlutterError error;
     try {
       await bundle.load('key');
     } on FlutterError catch (e) {
diff --git a/packages/flutter/test/services/autofill_test.dart b/packages/flutter/test/services/autofill_test.dart
index 5ce1eb4ee8..0078c4fc20 100644
--- a/packages/flutter/test/services/autofill_test.dart
+++ b/packages/flutter/test/services/autofill_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:convert' show utf8;
 
 import 'package:flutter/services.dart';
@@ -13,13 +11,12 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('TextInput message channels', () {
-    FakeTextChannel fakeTextChannel;
-    FakeAutofillScope scope;
+    late FakeTextChannel fakeTextChannel;
+    final FakeAutofillScope scope = FakeAutofillScope();
 
     setUp(() {
       fakeTextChannel = FakeTextChannel((MethodCall call) async {});
       TextInput.setChannel(fakeTextChannel);
-      scope ??= FakeAutofillScope();
       scope.clients.clear();
     });
 
@@ -28,35 +25,8 @@ void main() {
       TextInput.setChannel(SystemChannels.textInput);
     });
 
-    test('mandatory fields are mandatory', () async {
-      AutofillConfiguration config;
-      try {
-        config = AutofillConfiguration(
-          uniqueIdentifier: null,
-          autofillHints: const <String>['test'],
-          currentEditingValue: const TextEditingValue(),
-        );
-      } catch (e) {
-        expect(e.toString(), contains('uniqueIdentifier != null'));
-      }
-
-      expect(config, isNull);
-
-      try {
-        config = AutofillConfiguration(
-          uniqueIdentifier: 'id',
-          autofillHints: null,
-          currentEditingValue: const TextEditingValue(),
-        );
-      } catch (e) {
-        expect(e.toString(), contains('autofillHints != null'));
-      }
-
-      expect(config, isNull);
-    });
-
     test('throws if the hint list is empty', () async {
-      Map<String, dynamic> json;
+      Map<String, dynamic>? json;
       try {
         const AutofillConfiguration config = AutofillConfiguration(
           uniqueIdentifier: 'id',
@@ -113,7 +83,7 @@ void main() {
         ]);
 
         const TextEditingValue text2 = TextEditingValue(text: 'Text 2');
-        fakeTextChannel.incoming(MethodCall(
+        fakeTextChannel.incoming?.call(MethodCall(
           'TextInputClient.updateEditingStateWithTag',
           <dynamic>[0, <String, dynamic>{ client2.autofillId : text2.toJSON() }],
         ));
@@ -130,7 +100,7 @@ class FakeAutofillClient implements TextInputClient, AutofillClient {
   String get autofillId => hashCode.toString();
 
   @override
-  TextInputConfiguration textInputConfiguration;
+  late TextInputConfiguration textInputConfiguration;
 
   @override
   void updateEditingValue(TextEditingValue newEditingValue) {
@@ -139,7 +109,7 @@ class FakeAutofillClient implements TextInputClient, AutofillClient {
   }
 
   @override
-  AutofillScope currentAutofillScope;
+  AutofillScope? currentAutofillScope;
 
   String latestMethodCall = '';
 
@@ -179,7 +149,7 @@ class FakeAutofillScope with AutofillScopeMixin implements AutofillScope {
   Iterable<AutofillClient> get autofillClients => clients.values;
 
   @override
-  AutofillClient getAutofillClient(String autofillId) => clients[autofillId];
+  AutofillClient getAutofillClient(String autofillId) => clients[autofillId]!;
 
   void register(AutofillClient client) {
     clients.putIfAbsent(client.autofillId, () => client);
@@ -190,7 +160,7 @@ class FakeTextChannel implements MethodChannel {
   FakeTextChannel(this.outgoing) : assert(outgoing != null);
 
   Future<dynamic> Function(MethodCall) outgoing;
-  Future<void> Function(MethodCall) incoming;
+  Future<void> Function(MethodCall)? incoming;
 
   List<MethodCall> outgoingCalls = <MethodCall>[];
 
@@ -217,18 +187,18 @@ class FakeTextChannel implements MethodChannel {
   String get name => 'flutter/textinput';
 
   @override
-  void setMethodCallHandler(Future<void> Function(MethodCall call) handler) {
+  void setMethodCallHandler(Future<void> Function(MethodCall call)? handler) {
     incoming = handler;
   }
 
   @override
-  bool checkMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+  bool checkMethodCallHandler(Future<void> Function(MethodCall call)? handler) => throw UnimplementedError();
 
   @override
-  void setMockMethodCallHandler(Future<void> Function(MethodCall call) handler)  => throw UnimplementedError();
+  void setMockMethodCallHandler(Future<void> Function(MethodCall call)? handler)  => throw UnimplementedError();
 
   @override
-  bool checkMockMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+  bool checkMockMethodCallHandler(Future<void> Function(MethodCall call)? handler) => throw UnimplementedError();
 
   void validateOutgoingMethodCalls(List<MethodCall> calls) {
     expect(outgoingCalls.length, calls.length);
diff --git a/packages/flutter/test/services/binding_test.dart b/packages/flutter/test/services/binding_test.dart
index f5ed714f80..513eb3828e 100644
--- a/packages/flutter/test/services/binding_test.dart
+++ b/packages/flutter/test/services/binding_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:typed_data';
 
 import 'package:flutter/foundation.dart';
@@ -45,7 +43,7 @@ class TestBinding extends BindingBase with SchedulerBinding, ServicesBinding {
   @override
   BinaryMessenger createBinaryMessenger() {
     return super.createBinaryMessenger()
-      ..setMockMessageHandler('flutter/assets', (ByteData message) async {
+      ..setMockMessageHandler('flutter/assets', (ByteData? message) async {
         if (const StringCodec().decodeMessage(message) == 'NOTICES') {
           return const StringCodec().encodeMessage(licenses);
         }
diff --git a/packages/flutter/test/services/channel_buffers_test.dart b/packages/flutter/test/services/channel_buffers_test.dart
index ca9c8ab8eb..cbeb66095f 100644
--- a/packages/flutter/test/services/channel_buffers_test.dart
+++ b/packages/flutter/test/services/channel_buffers_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 // TODO(yjbanov): enable Web when https://github.com/flutter/engine/pull/12747 rolls into the framework.
 @TestOn('!chrome')
 
@@ -35,15 +33,15 @@ void main() {
     final TestChannelBuffersFlutterBinding binding = TestChannelBuffersFlutterBinding();
     expect(binding.defaultBinaryMessenger, isNotNull);
     bool didCallCallback = false;
-    final ui.PlatformMessageResponseCallback callback = (ByteData responseData) {
+    final ui.PlatformMessageResponseCallback callback = (ByteData? responseData) {
       didCallCallback = true;
     };
     const String payload = 'bar';
     final ByteData data = _makeByteData(payload);
     ui.channelBuffers.push(channel, data, callback);
     bool didDrainData = false;
-    binding.defaultBinaryMessenger.setMessageHandler(channel, (ByteData message) async {
-      expect(_getString(message), payload);
+    binding.defaultBinaryMessenger.setMessageHandler(channel, (ByteData? message) async {
+      expect(_getString(message!), payload);
       didDrainData = true;
       return null;
     });
diff --git a/packages/flutter/test/services/default_binary_messenger_test.dart b/packages/flutter/test/services/default_binary_messenger_test.dart
index 90af29409b..e90ecfb4aa 100644
--- a/packages/flutter/test/services/default_binary_messenger_test.dart
+++ b/packages/flutter/test/services/default_binary_messenger_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:convert';
 import 'dart:typed_data';
 import 'dart:ui' as ui;
@@ -23,22 +21,21 @@ void main() {
   test('default binary messenger calls callback once', () async {
     int count = 0;
     const String channel = 'foo';
-    ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
-        channel, _makeByteData('bar'), (ByteData message) async {
+    ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
+        channel, _makeByteData('bar'), (ByteData? message) async {
       count += 1;
     });
     expect(count, equals(0));
     await ui.channelBuffers.drain(channel,
-        (ByteData data, ui.PlatformMessageResponseCallback callback) {
+        (ByteData? data, ui.PlatformMessageResponseCallback callback) async {
       callback(null);
-      return null;
     });
     expect(count, equals(1));
   });
 
   test('can check the handler', () {
-    Future<ByteData> handler(ByteData call) => Future<ByteData>.value(null);
-    final BinaryMessenger messenger = ServicesBinding.instance.defaultBinaryMessenger;
+    Future<ByteData> handler(ByteData? call) => Future<ByteData>.value(null);
+    final BinaryMessenger messenger = ServicesBinding.instance!.defaultBinaryMessenger;
 
     expect(messenger.checkMessageHandler('test_channel', null), true);
     expect(messenger.checkMessageHandler('test_channel', handler), false);
@@ -48,8 +45,8 @@ void main() {
   });
 
   test('can check the mock handler', () {
-    Future<ByteData> handler(ByteData call) => Future<ByteData>.value(null);
-    final BinaryMessenger messenger = ServicesBinding.instance.defaultBinaryMessenger;
+    Future<ByteData> handler(ByteData? call) => Future<ByteData>.value(null);
+    final BinaryMessenger messenger = ServicesBinding.instance!.defaultBinaryMessenger;
 
     expect(messenger.checkMockMessageHandler('test_channel', null), true);
     expect(messenger.checkMockMessageHandler('test_channel', handler), false);
diff --git a/packages/flutter/test/services/fake_platform_views.dart b/packages/flutter/test/services/fake_platform_views.dart
index 2ca9e7d4ca..c105787c49 100644
--- a/packages/flutter/test/services/fake_platform_views.dart
+++ b/packages/flutter/test/services/fake_platform_views.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:async';
 import 'dart:typed_data';
 
@@ -14,10 +12,7 @@ import 'package:flutter/widgets.dart';
 
 /// Used in internal testing.
 class FakePlatformViewController extends PlatformViewController {
-
-  FakePlatformViewController(int id) {
-    _id = id;
-  }
+  FakePlatformViewController(this.viewId);
 
   bool disposed = false;
   bool focusCleared = false;
@@ -25,10 +20,8 @@ class FakePlatformViewController extends PlatformViewController {
   /// Events that are dispatched.
   List<PointerEvent> dispatchedPointerEvents = <PointerEvent>[];
 
-  int _id;
-
   @override
-  int get viewId => _id;
+  final int viewId;
 
   @override
   Future<void> dispatchPointerEvent(PointerEvent event) async {
@@ -66,7 +59,7 @@ class FakeAndroidViewController implements AndroidViewController {
   final int viewId;
 
   @override
-  Offset Function(Offset position) pointTransformer;
+  Offset Function(Offset position)? pointTransformer;
 
   @override
   Future<void> dispatchPointerEvent(PointerEvent event) async {
@@ -98,7 +91,7 @@ class FakeAndroidViewController implements AndroidViewController {
   int get textureId => throw UnimplementedError();
 
   @override
-  bool isCreated;
+  bool get isCreated => throw UnimplementedError();
 
   @override
   void addOnPlatformViewCreatedListener(PlatformViewCreatedCallback listener) =>
@@ -133,7 +126,6 @@ class FakeAndroidPlatformViewsController {
     SystemChannels.platform_views.setMockMethodCallHandler(_onMethodCall);
   }
 
-
   Iterable<FakeAndroidPlatformView> get views => _views.values;
   final Map<int, FakeAndroidPlatformView> _views = <int, FakeAndroidPlatformView>{};
 
@@ -143,11 +135,11 @@ class FakeAndroidPlatformViewsController {
 
   int _textureCounter = 0;
 
-  Completer<void> resizeCompleter;
+  Completer<void>? resizeCompleter;
 
-  Completer<void> createCompleter;
+  Completer<void>? createCompleter;
 
-  int lastClearedFocusViewId;
+  int? lastClearedFocusViewId;
 
   void registerViewType(String viewType) {
     _registeredViewTypes.add(viewType);
@@ -156,7 +148,8 @@ class FakeAndroidPlatformViewsController {
   void invokeViewFocused(int viewId) {
     final MethodCodec codec = SystemChannels.platform_views.codec;
     final ByteData data = codec.encodeMethodCall(MethodCall('viewFocused', viewId));
-    ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(SystemChannels.platform_views.name, data, (ByteData data) {});
+    ServicesBinding.instance!.defaultBinaryMessenger
+        .handlePlatformMessage(SystemChannels.platform_views.name, data, (ByteData? data) {});
   }
 
   Future<dynamic> _onMethodCall(MethodCall call) {
@@ -200,7 +193,7 @@ class FakeAndroidPlatformViewsController {
       );
 
     if (createCompleter != null) {
-      await createCompleter.future;
+      await createCompleter!.future;
     }
 
     _views[id] = FakeAndroidPlatformView(id, viewType,
@@ -219,9 +212,9 @@ class FakeAndroidPlatformViewsController {
     final int id = call.arguments['id'] as int;
     final bool hybrid = call.arguments['hybrid'] as bool;
 
-    if (hybrid && !_views[id].hybrid) {
+    if (hybrid && !_views[id]!.hybrid!) {
       throw ArgumentError('An $AndroidViewController using hybrid composition must pass `hybrid: true`');
-    } else if (!hybrid && _views[id].hybrid != null && _views[id].hybrid) {
+    } else if (!hybrid && (_views[id]!.hybrid ?? false)) {
       throw ArgumentError('An $AndroidViewController not using hybrid composition must pass `hybrid: false`');
     }
 
@@ -248,9 +241,9 @@ class FakeAndroidPlatformViewsController {
       );
 
     if (resizeCompleter != null) {
-      await resizeCompleter.future;
+      await resizeCompleter!.future;
     }
-    _views[id] = _views[id].copyWith(size: Size(width, height));
+    _views[id] = _views[id]!.copyWith(size: Size(width, height));
 
     return Future<dynamic>.sync(() => null);
   }
@@ -273,7 +266,7 @@ class FakeAndroidPlatformViewsController {
     if (!motionEvents.containsKey(id))
       motionEvents[id] = <FakeAndroidMotionEvent> [];
 
-    motionEvents[id].add(FakeAndroidMotionEvent(action, pointerIds, pointerOffsets));
+    motionEvents[id]!.add(FakeAndroidMotionEvent(action, pointerIds, pointerOffsets));
     return Future<dynamic>.sync(() => null);
   }
 
@@ -288,7 +281,7 @@ class FakeAndroidPlatformViewsController {
         message: 'Trying to resize a platform view with unknown id: $id',
       );
 
-    _views[id] = _views[id].copyWith(layoutDirection: layoutDirection);
+    _views[id] = _views[id]!.copyWith(layoutDirection: layoutDirection);
 
     return Future<dynamic>.sync(() => null);
   }
@@ -312,7 +305,6 @@ class FakeIosPlatformViewsController {
     SystemChannels.platform_views.setMockMethodCallHandler(_onMethodCall);
   }
 
-
   Iterable<FakeUiKitView> get views => _views.values;
   final Map<int, FakeUiKitView> _views = <int, FakeUiKitView>{};
 
@@ -320,7 +312,7 @@ class FakeIosPlatformViewsController {
 
   // When this completer is non null, the 'create' method channel call will be
   // delayed until it completes.
-  Completer<void> creationDelay;
+  Completer<void>? creationDelay;
 
   // Maps a view id to the number of gestures it accepted so far.
   final Map<int, int> gesturesAccepted = <int, int>{};
@@ -348,7 +340,7 @@ class FakeIosPlatformViewsController {
 
   Future<dynamic> _create(MethodCall call) async {
     if (creationDelay != null)
-      await creationDelay.future;
+      await creationDelay!.future;
     final Map<dynamic, dynamic> args = call.arguments as Map<dynamic, dynamic>;
     final int id = args['id'] as int;
     final String viewType = args['viewType'] as String;
@@ -371,21 +363,21 @@ class FakeIosPlatformViewsController {
     _views[id] = FakeUiKitView(id, viewType, creationParams);
     gesturesAccepted[id] = 0;
     gesturesRejected[id] = 0;
-    return Future<int>.sync(() => null);
+    return Future<int?>.sync(() => null);
   }
 
   Future<dynamic> _acceptGesture(MethodCall call) async {
     final Map<dynamic, dynamic> args = call.arguments as Map<dynamic, dynamic>;
     final int id = args['id'] as int;
-    gesturesAccepted[id] += 1;
-    return Future<int>.sync(() => null);
+    gesturesAccepted[id] = gesturesAccepted[id]! + 1;
+    return Future<int?>.sync(() => null);
   }
 
   Future<dynamic> _rejectGesture(MethodCall call) async {
     final Map<dynamic, dynamic> args = call.arguments as Map<dynamic, dynamic>;
     final int id = args['id'] as int;
-    gesturesRejected[id] += 1;
-    return Future<int>.sync(() => null);
+    gesturesRejected[id] = gesturesRejected[id]! + 1;
+    return Future<int?>.sync(() => null);
   }
 
   Future<dynamic> _dispose(MethodCall call) {
@@ -413,9 +405,9 @@ class FakeHtmlPlatformViewsController {
 
   final Set<String> _registeredViewTypes = <String>{};
 
-  Completer<void> resizeCompleter;
+  late Completer<void> resizeCompleter;
 
-  Completer<void> createCompleter;
+  Completer<void>? createCompleter;
 
   void registerViewType(String viewType) {
     _registeredViewTypes.add(viewType);
@@ -449,11 +441,11 @@ class FakeHtmlPlatformViewsController {
       );
 
     if (createCompleter != null) {
-      await createCompleter.future;
+      await createCompleter!.future;
     }
 
     _views[id] = FakeHtmlPlatformView(id, viewType);
-    return Future<int>.sync(() => null);
+    return Future<int?>.sync(() => null);
   }
 
   Future<dynamic> _dispose(MethodCall call) {
@@ -476,12 +468,12 @@ class FakeAndroidPlatformView {
 
   final int id;
   final String type;
-  final Uint8List creationParams;
-  final Size size;
+  final Uint8List? creationParams;
+  final Size? size;
   final int layoutDirection;
-  final bool hybrid;
+  final bool? hybrid;
 
-  FakeAndroidPlatformView copyWith({Size size, int layoutDirection}) => FakeAndroidPlatformView(
+  FakeAndroidPlatformView copyWith({Size? size, int? layoutDirection}) => FakeAndroidPlatformView(
     id,
     type,
     size ?? this.size,
@@ -544,7 +536,7 @@ class FakeUiKitView {
 
   final int id;
   final String type;
-  final Uint8List creationParams;
+  final Uint8List? creationParams;
 
   @override
   bool operator ==(Object other) {
diff --git a/packages/flutter/test/services/font_loader_test.dart b/packages/flutter/test/services/font_loader_test.dart
index 69517b2e32..8eb08520e8 100644
--- a/packages/flutter/test/services/font_loader_test.dart
+++ b/packages/flutter/test/services/font_loader_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:typed_data';
 
 import 'package:flutter/services.dart';
diff --git a/packages/flutter/test/services/haptic_feedback_test.dart b/packages/flutter/test/services/haptic_feedback_test.dart
index 1da74c2dc6..6234a97579 100644
--- a/packages/flutter/test/services/haptic_feedback_test.dart
+++ b/packages/flutter/test/services/haptic_feedback_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
diff --git a/packages/flutter/test/services/keyboard_key_test.dart b/packages/flutter/test/services/keyboard_key_test.dart
index f4d0ceeebc..40abb9c26e 100644
--- a/packages/flutter/test/services/keyboard_key_test.dart
+++ b/packages/flutter/test/services/keyboard_key_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
diff --git a/packages/flutter/test/services/lifecycle_test.dart b/packages/flutter/test/services/lifecycle_test.dart
index cb31e73d16..4a5524c482 100644
--- a/packages/flutter/test/services/lifecycle_test.dart
+++ b/packages/flutter/test/services/lifecycle_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/services.dart';
 
@@ -11,7 +9,7 @@ void main() {
   testWidgets('initialLifecycleState is used to init state paused', (WidgetTester tester) async {
     // The lifecycleState is null initially in tests as there is no
     // initialLifecycleState.
-    expect(ServicesBinding.instance.lifecycleState, equals(null));
+    expect(ServicesBinding.instance!.lifecycleState, equals(null));
     // Mock the Window to provide paused as the AppLifecycleState
     final TestWidgetsFlutterBinding binding = tester.binding;
     // Use paused as the initial state.
@@ -20,6 +18,6 @@ void main() {
 
     // The lifecycleState should now be the state we passed above,
     // even though no lifecycle event was fired from the platform.
-    expect(ServicesBinding.instance.lifecycleState.toString(), equals('AppLifecycleState.paused'));
+    expect(ServicesBinding.instance!.lifecycleState.toString(), equals('AppLifecycleState.paused'));
   });
 }
diff --git a/packages/flutter/test/services/message_codecs_test.dart b/packages/flutter/test/services/message_codecs_test.dart
index 46c966060f..b4c1394ea6 100644
--- a/packages/flutter/test/services/message_codecs_test.dart
+++ b/packages/flutter/test/services/message_codecs_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 // This files contains message codec tests that are supported both on the Web
 // and in the VM. For VM-only tests see message_codecs_vm_test.dart.
 
@@ -18,26 +16,25 @@ import 'message_codecs_testing.dart';
 
 void main() {
   group('Binary codec', () {
-    const MessageCodec<ByteData> binary = BinaryCodec();
+    const MessageCodec<ByteData?> binary = BinaryCodec();
     test('should encode and decode simple messages', () {
-      checkEncodeDecode<ByteData>(binary, null);
-      checkEncodeDecode<ByteData>(binary, ByteData(0));
-      checkEncodeDecode<ByteData>(binary, ByteData(4)..setInt32(0, -7));
+      checkEncodeDecode<ByteData?>(binary, null);
+      checkEncodeDecode<ByteData?>(binary, ByteData(0));
+      checkEncodeDecode<ByteData?>(binary, ByteData(4)..setInt32(0, -7));
     });
   });
   group('String codec', () {
-    const MessageCodec<String> string = StringCodec();
+    const MessageCodec<String?> string = StringCodec();
     test('should encode and decode simple messages', () {
-      checkEncodeDecode<String>(string, null);
-      checkEncodeDecode<String>(string, '');
-      checkEncodeDecode<String>(string, 'hello');
-      checkEncodeDecode<String>(string, 'special chars >\u263A\u{1F602}<');
+      checkEncodeDecode<String?>(string, null);
+      checkEncodeDecode<String?>(string, '');
+      checkEncodeDecode<String?>(string, 'hello');
+      checkEncodeDecode<String?>(string, 'special chars >\u263A\u{1F602}<');
     });
     test('ByteData with offset', () {
-      const MessageCodec<String> string = StringCodec();
-      final ByteData helloWorldByteData = string.encodeMessage('hello world');
-      final ByteData helloByteData = string.encodeMessage('hello');
-
+      const MessageCodec<String?> string = StringCodec();
+      final ByteData helloWorldByteData = string.encodeMessage('hello world')!;
+      final ByteData helloByteData = string.encodeMessage('hello')!;
       final ByteData offsetByteData = ByteData.view(
         helloWorldByteData.buffer,
         helloByteData.lengthInBytes,
@@ -97,7 +94,7 @@ void main() {
               e.details == 'errorDetails')));
     });
     test('should decode error envelope with native stacktrace.', () {
-      final ByteData errorData = stringCodec.encodeMessage(json
+      final ByteData? errorData = stringCodec.encodeMessage(json
           .encode(<dynamic>[
         'errorCode',
         'errorMessage',
@@ -105,7 +102,7 @@ void main() {
         'errorStacktrace'
       ]));
       expect(
-          () => jsonMethodCodec.decodeEnvelope(errorData),
+          () => jsonMethodCodec.decodeEnvelope(errorData!),
           throwsA(predicate((PlatformException e) =>
               e is PlatformException && e.stacktrace == 'errorStacktrace')));
     });
diff --git a/packages/flutter/test/services/message_codecs_testing.dart b/packages/flutter/test/services/message_codecs_testing.dart
index 8d95b7e55a..26e3edd298 100644
--- a/packages/flutter/test/services/message_codecs_testing.dart
+++ b/packages/flutter/test/services/message_codecs_testing.dart
@@ -23,9 +23,9 @@ void checkEncodeDecode<T>(MessageCodec<T> codec, T message) {
     expect(decoded, isNull);
   } else {
     expect(deepEquals(message, decoded), isTrue);
-    final ByteData encodedAgain = codec.encodeMessage(decoded)!;
+    final ByteData? encodedAgain = codec.encodeMessage(decoded);
     expect(
-      encodedAgain.buffer.asUint8List(),
+      encodedAgain!.buffer.asUint8List(),
       orderedEquals(encoded!.buffer.asUint8List()),
     );
   }
diff --git a/packages/flutter/test/services/message_codecs_vm_test.dart b/packages/flutter/test/services/message_codecs_vm_test.dart
index 33bccf5501..aaaba050e8 100644
--- a/packages/flutter/test/services/message_codecs_vm_test.dart
+++ b/packages/flutter/test/services/message_codecs_vm_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 // This file contains tests that are only supported by the Dart VM. For
 // example, on the Web there's no way to express large integers.
diff --git a/packages/flutter/test/services/platform_channel_test.dart b/packages/flutter/test/services/platform_channel_test.dart
index 5fddebdb68..a119b1c988 100644
--- a/packages/flutter/test/services/platform_channel_test.dart
+++ b/packages/flutter/test/services/platform_channel_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:typed_data';
 
 import 'package:flutter/services.dart';
@@ -15,24 +13,24 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('BasicMessageChannel', () {
-    const MessageCodec<String> string = StringCodec();
-    const BasicMessageChannel<String> channel = BasicMessageChannel<String>('ch', string);
+    const MessageCodec<String?> string = StringCodec();
+    const BasicMessageChannel<String?> channel = BasicMessageChannel<String?>('ch', string);
     test('can send string message and get reply', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch',
-        (ByteData message) async => string.encodeMessage(string.decodeMessage(message) + ' world'),
+        (ByteData? message) async => string.encodeMessage(string.decodeMessage(message)! + ' world'),
       );
-      final String reply = await channel.send('hello');
+      final String? reply = await channel.send('hello');
       expect(reply, equals('hello world'));
     });
 
     test('can receive string message and send reply', () async {
-      channel.setMessageHandler((String message) async => message + ' world');
-      String reply;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      channel.setMessageHandler((String? message) async => message! + ' world');
+      String? reply;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'ch',
         const StringCodec().encodeMessage('hello'),
-        (ByteData replyBinary) {
+        (ByteData? replyBinary) {
           reply = string.decodeMessage(replyBinary);
         },
       );
@@ -46,9 +44,9 @@ void main() {
     const MethodChannel channel = MethodChannel('ch7', jsonMethod);
     const OptionalMethodChannel optionalMethodChannel = OptionalMethodChannel('ch8', jsonMethod);
     test('can invoke method and get result', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'sayHello') {
             return jsonMessage.encodeMessage(<dynamic>['${methodCall['args']} world']);
@@ -57,14 +55,14 @@ void main() {
           }
         },
       );
-      final String result = await channel.invokeMethod('sayHello', 'hello');
+      final String? result = await channel.invokeMethod('sayHello', 'hello');
       expect(result, equals('hello world'));
     });
 
     test('can invoke list method and get result', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'sayHello') {
             return jsonMessage.encodeMessage(<dynamic>[<String>['${methodCall['args']}', 'world']]);
@@ -78,9 +76,9 @@ void main() {
     });
 
     test('can invoke list method and get null result', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'sayHello') {
             return jsonMessage.encodeMessage(<dynamic>[null]);
@@ -93,9 +91,9 @@ void main() {
     });
 
     test('can invoke map method and get result', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'sayHello') {
             return jsonMessage.encodeMessage(<dynamic>[<String, String>{'${methodCall['args']}': 'world'}]);
@@ -109,9 +107,9 @@ void main() {
     });
 
     test('can invoke map method and get null result', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'sayHello') {
             return jsonMessage.encodeMessage(<dynamic>[null]);
@@ -124,9 +122,9 @@ void main() {
     });
 
     test('can invoke method and get error', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async {
+        (ByteData? message) async {
           return jsonMessage.encodeMessage(<dynamic>[
             'bad',
             'Something happened',
@@ -147,9 +145,9 @@ void main() {
     });
 
     test('can invoke unimplemented method', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch7',
-        (ByteData message) async => null,
+        (ByteData? message) async => null,
       );
       try {
         await channel.invokeMethod<void>('sayHello', 'hello');
@@ -163,19 +161,19 @@ void main() {
     });
 
     test('can invoke unimplemented method (optional)', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch8',
-        (ByteData message) async => null,
+        (ByteData? message) async => null,
       );
-      final String result = await optionalMethodChannel.invokeMethod<String>('sayHello', 'hello');
+      final String? result = await optionalMethodChannel.invokeMethod<String>('sayHello', 'hello');
       expect(result, isNull);
     });
 
     test('can handle method call with no registered plugin (setting before)', () async {
       channel.setMethodCallHandler(null);
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
-      ByteData envelope;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+      ByteData? envelope;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData? result) {
         envelope = result;
       });
       await null; // just in case there's something async happening
@@ -184,8 +182,8 @@ void main() {
 
     test('can handle method call with no registered plugin (setting after)', () async {
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
-      ByteData envelope;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+      ByteData? envelope;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData? result) {
         envelope = result;
       });
       channel.setMethodCallHandler(null);
@@ -198,8 +196,8 @@ void main() {
         throw MissingPluginException();
       });
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
-      ByteData envelope;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+      ByteData? envelope;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData? result) {
         envelope = result;
       });
       expect(envelope, isNull);
@@ -208,11 +206,11 @@ void main() {
     test('can handle method call with successful result', () async {
       channel.setMethodCallHandler((MethodCall call) async => '${call.arguments}, world');
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
-      ByteData envelope;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+      ByteData? envelope;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData? result) {
         envelope = result;
       });
-      expect(jsonMethod.decodeEnvelope(envelope), equals('hello, world'));
+      expect(jsonMethod.decodeEnvelope(envelope!), equals('hello, world'));
     });
 
     test('can handle method call with expressive error result', () async {
@@ -220,12 +218,12 @@ void main() {
         throw PlatformException(code: 'bad', message: 'sayHello failed', details: null);
       });
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
-      ByteData envelope;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+      ByteData? envelope;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData? result) {
         envelope = result;
       });
       try {
-        jsonMethod.decodeEnvelope(envelope);
+        jsonMethod.decodeEnvelope(envelope!);
         fail('Exception expected');
       } on PlatformException catch (e) {
         expect(e.code, equals('bad'));
@@ -240,12 +238,12 @@ void main() {
         throw ArgumentError('bad');
       });
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
-      ByteData envelope;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+      ByteData? envelope;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData? result) {
         envelope = result;
       });
       try {
-        jsonMethod.decodeEnvelope(envelope);
+        jsonMethod.decodeEnvelope(envelope!);
         fail('Exception expected');
       } on PlatformException catch (e) {
         expect(e.code, equals('error'));
@@ -280,18 +278,18 @@ void main() {
     const MessageCodec<dynamic> jsonMessage = JSONMessageCodec();
     const MethodCodec jsonMethod = JSONMethodCodec();
     const EventChannel channel = EventChannel('ch', jsonMethod);
-    void emitEvent(ByteData event) {
-      ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+    void emitEvent(ByteData? event) {
+      ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'ch',
         event,
-        (ByteData reply) { },
+        (ByteData? reply) {},
       );
     }
     test('can receive event stream', () async {
       bool canceled = false;
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'listen') {
             final String argument = methodCall['args'] as String;
@@ -314,9 +312,9 @@ void main() {
     });
 
     test('can receive error event', () async {
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(
         'ch',
-        (ByteData message) async {
+        (ByteData? message) async {
           final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message) as Map<dynamic, dynamic>;
           if (methodCall['method'] == 'listen') {
             final String argument = methodCall['args'] as String;
diff --git a/packages/flutter/test/services/platform_messages_test.dart b/packages/flutter/test/services/platform_messages_test.dart
index 2000d594a5..43a9803768 100644
--- a/packages/flutter/test/services/platform_messages_test.dart
+++ b/packages/flutter/test/services/platform_messages_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 @TestOn('!chrome') // missing web infrastructure for plugins.
 
@@ -16,20 +15,20 @@ void main() {
     // Initialize all bindings because defaultBinaryMessenger.send() needs a window.
     TestWidgetsFlutterBinding.ensureInitialized();
 
-    final List<ByteData> log = <ByteData>[];
+    final List<ByteData?> log = <ByteData>[];
 
-    ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler('test1', (ByteData message) async {
+    ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler('test1', (ByteData? message) async {
       log.add(message);
       return null;
     });
 
     final ByteData message = ByteData(2)..setUint16(0, 0xABCD);
-    await ServicesBinding.instance.defaultBinaryMessenger.send('test1', message);
+    await ServicesBinding.instance!.defaultBinaryMessenger.send('test1', message);
     expect(log, equals(<ByteData>[message]));
     log.clear();
 
-    ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler('test1', null);
-    await ServicesBinding.instance.defaultBinaryMessenger.send('test1', message);
+    ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler('test1', null);
+    await ServicesBinding.instance!.defaultBinaryMessenger.send('test1', message);
     expect(log, isEmpty);
   });
 }
diff --git a/packages/flutter/test/services/platform_views_test.dart b/packages/flutter/test/services/platform_views_test.dart
index 9ab321baa3..1db8c6adcb 100644
--- a/packages/flutter/test/services/platform_views_test.dart
+++ b/packages/flutter/test/services/platform_views_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/painting.dart';
 import 'package:flutter/services.dart';
@@ -15,7 +14,7 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('Android', () {
-    FakeAndroidPlatformViewsController viewsController;
+    late FakeAndroidPlatformViewsController viewsController;
     setUp(() {
       viewsController = FakeAndroidPlatformViewsController();
     });
@@ -202,7 +201,7 @@ void main() {
   });
 
   group('iOS', () {
-    FakeIosPlatformViewsController viewsController;
+    late FakeIosPlatformViewsController viewsController;
     setUp(() {
       viewsController = FakeIosPlatformViewsController();
     });
diff --git a/packages/flutter/test/services/raw_keyboard_test.dart b/packages/flutter/test/services/raw_keyboard_test.dart
index b138267c51..87fe0edfa8 100644
--- a/packages/flutter/test/services/raw_keyboard_test.dart
+++ b/packages/flutter/test/services/raw_keyboard_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
@@ -209,10 +208,10 @@ void main() {
       // when this event is received, but it's not in keysPressed yet.
       data['modifiers'] |= RawKeyEventDataMacOs.modifierLeftShift | RawKeyEventDataMacOs.modifierShift;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {},
+        (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -234,10 +233,10 @@ void main() {
       // when this event is received, but it's not in keysPressed yet.
       data['modifiers'] |= RawKeyEventDataWindows.modifierLeftShift | RawKeyEventDataWindows.modifierShift;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {},
+        (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -259,10 +258,10 @@ void main() {
       // when this event is received, but it's not in keysPressed yet.
       data['metaState'] |= RawKeyEventDataAndroid.modifierLeftShift | RawKeyEventDataAndroid.modifierShift;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {},
+        (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -284,10 +283,10 @@ void main() {
       // when this event is received, but it's not in keysPressed yet.
       data['modifiers'] |= RawKeyEventDataFuchsia.modifierLeftShift;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {},
+        (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -309,10 +308,10 @@ void main() {
       // when this event is received, but it's not in keysPressed yet.
       data['modifiers'] |= GLFWKeyHelper.modifierShift;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {},
+        (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -343,10 +342,10 @@ void main() {
         RawKeyEventDataAndroid.modifierControl |
         RawKeyEventDataAndroid.modifierMeta;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-            (ByteData data) {},
+            (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -381,10 +380,10 @@ void main() {
           RawKeyEventDataMacOs.modifierCommand |
           RawKeyEventDataMacOs.modifierControl;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-            (ByteData data) {},
+            (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -419,10 +418,10 @@ void main() {
           RawKeyEventDataWindows.modifierAlt |
           RawKeyEventDataWindows.modifierControl;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-            (ByteData data) {},
+            (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -456,10 +455,10 @@ void main() {
         GLFWKeyHelper.modifierControl |
         GLFWKeyHelper.modifierMeta;
       // dispatch the modified data.
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-            (ByteData data) {},
+            (ByteData? data) {},
       );
       expect(
         RawKeyboard.instance.keysPressed,
@@ -480,13 +479,13 @@ void main() {
     });
 
     testWidgets('RawKeyboard asserts if no keys are in keysPressed after receiving a key down event', (WidgetTester tester) async {
-      FlutterErrorDetails errorDetails;
-      final FlutterExceptionHandler oldHandler = FlutterError.onError;
+      FlutterErrorDetails? errorDetails;
+      final FlutterExceptionHandler? oldHandler = FlutterError.onError;
       FlutterError.onError = (FlutterErrorDetails details) {
         errorDetails = details;
       };
       try {
-        await ServicesBinding.instance.defaultBinaryMessenger
+        await ServicesBinding.instance!.defaultBinaryMessenger
             .handlePlatformMessage(
           SystemChannels.keyEvent.name,
           SystemChannels.keyEvent.codec.encodeMessage(const <String, dynamic>{
@@ -498,13 +497,13 @@ void main() {
             'source': 0x101,
             'deviceId': 1,
           }),
-              (ByteData data) {},
+              (ByteData? data) {},
         );
       } finally {
         FlutterError.onError = oldHandler;
       }
       expect(errorDetails, isNotNull);
-      expect(errorDetails.stack, isNotNull);
+      expect(errorDetails!.stack, isNotNull);
       final String fullErrorMessage = errorDetails.toString().replaceAll('\n', ' ');
       expect(fullErrorMessage, contains('Attempted to send a key down event when no keys are in keysPressed'));
     });
@@ -542,16 +541,16 @@ void main() {
         });
         final RawKeyEventDataAndroid data = event.data as RawKeyEventDataAndroid;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key) {
+          if (modifierTests[modifier]!.key == key) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: "$key should be pressed with metaState $modifier, but isn't.",
             );
-            expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+            expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier.',
             );
@@ -578,21 +577,21 @@ void main() {
         });
         final RawKeyEventDataAndroid data = event.data as RawKeyEventDataAndroid;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key || key == ModifierKey.functionModifier) {
+          if (modifierTests[modifier]!.key == key || key == ModifierKey.functionModifier) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: '$key should be pressed with metaState $modifier '
                   "and additional key ${RawKeyEventDataAndroid.modifierFunction}, but isn't.",
             );
             if (key != ModifierKey.functionModifier) {
-              expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+              expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
             } else {
               expect(data.getModifierSide(key), equals(KeyboardSide.all));
             }
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier '
                   'and additional key ${RawKeyEventDataAndroid.modifierFunction}.',
@@ -741,11 +740,11 @@ void main() {
         platform: 'android',
         isDown: true,
       );
-      Map<String, dynamic> message;
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      Map<String, dynamic>? message;
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {
+        (ByteData? data) {
           message = SystemChannels.keyEvent.codec.decodeMessage(data) as Map<String, dynamic>;
         },
       );
@@ -765,15 +764,15 @@ void main() {
       focusNode.requestFocus();
       await tester.pump();
 
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         SystemChannels.keyEvent.name,
         SystemChannels.keyEvent.codec.encodeMessage(data),
-        (ByteData data) {
+        (ByteData? data) {
           message = SystemChannels.keyEvent.codec.decodeMessage(data) as Map<String, dynamic>;
         },
       );
       expect(message, equals(<String, dynamic>{ 'handled': true }));
-      ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(SystemChannels.keyEvent.name, null);
+      ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler(SystemChannels.keyEvent.name, null);
     });
   });
   group('RawKeyEventDataFuchsia', () {
@@ -804,15 +803,15 @@ void main() {
         });
         final RawKeyEventDataFuchsia data = event.data as RawKeyEventDataFuchsia;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key) {
+          if (modifierTests[modifier]!.key == key) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: "$key should be pressed with metaState $modifier, but isn't.",
             );
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier.',
             );
@@ -835,16 +834,16 @@ void main() {
         });
         final RawKeyEventDataFuchsia data = event.data as RawKeyEventDataFuchsia;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key || key == ModifierKey.capsLockModifier) {
+          if (modifierTests[modifier]!.key == key || key == ModifierKey.capsLockModifier) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: '$key should be pressed with metaState $modifier '
                   "and additional key ${RawKeyEventDataFuchsia.modifierCapsLock}, but isn't.",
             );
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier '
                   'and additional key ${RawKeyEventDataFuchsia.modifierCapsLock}.',
@@ -920,16 +919,16 @@ void main() {
         });
         final RawKeyEventDataMacOs data = event.data as RawKeyEventDataMacOs;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key) {
+          if (modifierTests[modifier]!.key == key) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: "$key should be pressed with metaState $modifier, but isn't.",
             );
-            expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+            expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier.',
             );
@@ -954,21 +953,21 @@ void main() {
         });
         final RawKeyEventDataMacOs data = event.data as RawKeyEventDataMacOs;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key || key == ModifierKey.capsLockModifier) {
+          if (modifierTests[modifier]!.key == key || key == ModifierKey.capsLockModifier) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: '$key should be pressed with metaState $modifier '
                   "and additional key ${RawKeyEventDataMacOs.modifierCapsLock}, but isn't.",
             );
             if (key != ModifierKey.capsLockModifier) {
-              expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+              expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
             } else {
               expect(data.getModifierSide(key), equals(KeyboardSide.all));
             }
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier '
                   'and additional key ${RawKeyEventDataMacOs.modifierCapsLock}.',
@@ -1068,16 +1067,16 @@ void main() {
         });
         final RawKeyEventDataWindows data = event.data as RawKeyEventDataWindows;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key) {
+          if (modifierTests[modifier]!.key == key) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: "$key should be pressed with modifier $modifier, but isn't.",
             );
-            expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+            expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier.',
             );
@@ -1101,21 +1100,21 @@ void main() {
         });
         final RawKeyEventDataWindows data = event.data as RawKeyEventDataWindows;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key || key == ModifierKey.capsLockModifier) {
+          if (modifierTests[modifier]!.key == key || key == ModifierKey.capsLockModifier) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: '$key should be pressed with metaState $modifier '
                   "and additional key ${RawKeyEventDataWindows.modifierCaps}, but isn't.",
             );
             if (key != ModifierKey.capsLockModifier) {
-              expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+              expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
             } else {
               expect(data.getModifierSide(key), equals(KeyboardSide.all));
             }
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier '
                   'and additional key ${RawKeyEventDataWindows.modifierCaps}.',
@@ -1194,7 +1193,7 @@ void main() {
     };
 
     // How modifiers are interpreted depends upon the keyCode for GLFW.
-    int keyCodeForModifier(int modifier, {bool isLeft}) {
+    int keyCodeForModifier(int modifier, {required bool isLeft}) {
       switch (modifier) {
         case GLFWKeyHelper.modifierAlt:
           return isLeft ? 342 : 346;
@@ -1229,16 +1228,16 @@ void main() {
             });
             final RawKeyEventDataLinux data = event.data as RawKeyEventDataLinux;
             for (final ModifierKey key in ModifierKey.values) {
-              if (modifierTests[modifier].key == key) {
+              if (modifierTests[modifier]!.key == key) {
                 expect(
-                  data.isModifierPressed(key, side: modifierTests[modifier].side),
+                  data.isModifierPressed(key, side: modifierTests[modifier]!.side),
                   isDown ? isTrue : isFalse,
                   reason: "${isLeft ? 'left' : 'right'} $key ${isDown ? 'should' : 'should not'} be pressed with metaState $modifier, when key is ${isDown ? 'down' : 'up'}, but isn't.",
                 );
-                expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+                expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
               } else {
                 expect(
-                  data.isModifierPressed(key, side: modifierTests[modifier].side),
+                  data.isModifierPressed(key, side: modifierTests[modifier]!.side),
                   isFalse,
                   reason: "${isLeft ? 'left' : 'right'} $key should not be pressed with metaState $modifier, wwhen key is ${isDown ? 'down' : 'up'}, but is.",
                 );
@@ -1265,21 +1264,21 @@ void main() {
         });
         final RawKeyEventDataLinux data = event.data as RawKeyEventDataLinux;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key || key == ModifierKey.controlModifier) {
+          if (modifierTests[modifier]!.key == key || key == ModifierKey.controlModifier) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: '$key should be pressed with metaState $modifier '
                   "and additional key ${GLFWKeyHelper.modifierControl}, but isn't.",
             );
             if (key != ModifierKey.controlModifier) {
-              expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+              expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
             } else {
               expect(data.getModifierSide(key), equals(KeyboardSide.all));
             }
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier '
                   'and additional key ${GLFWKeyHelper.modifierControl}.',
@@ -1378,7 +1377,7 @@ void main() {
     };
 
     // How modifiers are interpreted depends upon the keyCode for GTK.
-    int keyCodeForModifier(int modifier, {bool isLeft}) {
+    int keyCodeForModifier(int modifier, {required bool isLeft}) {
       switch (modifier) {
         case GtkKeyHelper.modifierMod1:
           return isLeft ? 65513 : 65513;
@@ -1413,16 +1412,16 @@ void main() {
             });
             final RawKeyEventDataLinux data = event.data as RawKeyEventDataLinux;
             for (final ModifierKey key in ModifierKey.values) {
-              if (modifierTests[modifier].key == key) {
+              if (modifierTests[modifier]!.key == key) {
                 expect(
-                  data.isModifierPressed(key, side: modifierTests[modifier].side),
+                  data.isModifierPressed(key, side: modifierTests[modifier]!.side),
                   isDown ? isTrue : isFalse,
                   reason: "${isLeft ? 'left' : 'right'} $key ${isDown ? 'should' : 'should not'} be pressed with metaState $modifier, when key is ${isDown ? 'down' : 'up'}, but isn't.",
                 );
-                expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+                expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
               } else {
                 expect(
-                  data.isModifierPressed(key, side: modifierTests[modifier].side),
+                  data.isModifierPressed(key, side: modifierTests[modifier]!.side),
                   isFalse,
                   reason: "${isLeft ? 'left' : 'right'} $key should not be pressed with metaState $modifier, wwhen key is ${isDown ? 'down' : 'up'}, but is.",
                 );
@@ -1449,21 +1448,21 @@ void main() {
         });
         final RawKeyEventDataLinux data = event.data as RawKeyEventDataLinux;
         for (final ModifierKey key in ModifierKey.values) {
-          if (modifierTests[modifier].key == key || key == ModifierKey.controlModifier) {
+          if (modifierTests[modifier]!.key == key || key == ModifierKey.controlModifier) {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isTrue,
               reason: '$key should be pressed with metaState $modifier '
                   "and additional key ${GtkKeyHelper.modifierControl}, but isn't.",
             );
             if (key != ModifierKey.controlModifier) {
-              expect(data.getModifierSide(key), equals(modifierTests[modifier].side));
+              expect(data.getModifierSide(key), equals(modifierTests[modifier]!.side));
             } else {
               expect(data.getModifierSide(key), equals(KeyboardSide.all));
             }
           } else {
             expect(
-              data.isModifierPressed(key, side: modifierTests[modifier].side),
+              data.isModifierPressed(key, side: modifierTests[modifier]!.side),
               isFalse,
               reason: '$key should not be pressed with metaState $modifier '
                   'and additional key ${GtkKeyHelper.modifierControl}.',
diff --git a/packages/flutter/test/services/restoration_bucket_test.dart b/packages/flutter/test/services/restoration_bucket_test.dart
index 4d9051a591..f64883b59f 100644
--- a/packages/flutter/test/services/restoration_bucket_test.dart
+++ b/packages/flutter/test/services/restoration_bucket_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
@@ -54,7 +53,7 @@ void main() {
     expect(manager.updateScheduled, isFalse);
 
     // Can store null.
-    bucket.write<bool>('value4', null);
+    bucket.write<bool?>('value4', null);
     expect(manager.updateScheduled, isTrue);
     expect(bucket.read<int>('value4'), null);
     manager.doSerialization();
@@ -111,7 +110,7 @@ void main() {
     expect(manager.updateScheduled, isFalse);
 
     // Can store null.
-    child.write<bool>('value4', null);
+    child.write<bool?>('value4', null);
     expect(manager.updateScheduled, isTrue);
     expect(child.read<int>('value4'), null);
     manager.doSerialization();
@@ -327,7 +326,7 @@ void main() {
     final RestorationBucket root = RestorationBucket.root(manager: manager, rawData: rawData);
 
     final RestorationBucket child = root.claimChild('child1', debugOwner: 'owner1');
-    final Object rawChildData = rawData[childrenMapKey]['child1'];
+    final Object rawChildData = rawData[childrenMapKey]['child1'] as Object;
     expect(rawChildData, isNotNull);
 
     expect(manager.updateScheduled, isFalse);
@@ -370,9 +369,9 @@ void main() {
     final RestorationBucket child2 = root.claimChild('child2', debugOwner: 'owner1');
     manager.doSerialization();
 
-    final Object rawChild1Data = rawData[childrenMapKey]['child1'];
+    final Object rawChild1Data = rawData[childrenMapKey]['child1'] as Object;
     expect(rawChild1Data, isNotNull);
-    final Object rawChild2Data = rawData[childrenMapKey]['child2'];
+    final Object rawChild2Data = rawData[childrenMapKey]['child2'] as Object;
     expect(rawChild2Data, isNotNull);
 
     expect(child1.restorationId, 'child1');
@@ -396,7 +395,7 @@ void main() {
     final Map<String, dynamic> rawData = _createRawDataSet();
     final RestorationBucket root = RestorationBucket.root(manager: manager, rawData: rawData);
 
-    final Object rawChild1Data = rawData[childrenMapKey]['child1'];
+    final Object rawChild1Data = rawData[childrenMapKey]['child1'] as Object;
     expect(rawChild1Data, isNotNull);
 
     final RestorationBucket child1 = root.claimChild('child1', debugOwner: 'owner1');
@@ -462,7 +461,7 @@ void main() {
     manager.doSerialization();
     expect(manager.updateScheduled, isFalse);
 
-    final Object childOfChildData = rawData[childrenMapKey]['child1'][childrenMapKey]['childOfChild'];
+    final Object childOfChildData = rawData[childrenMapKey]['child1'][childrenMapKey]['childOfChild'] as Object;
     expect(childOfChildData, isNotEmpty);
 
     root.adoptChild(childOfChild);
@@ -497,7 +496,7 @@ void main() {
     final RestorationBucket childOfChild = child.claimChild('child1', debugOwner: 'owner2');
     childOfChild.write<String>('foo', 'bar');
 
-    final Object childOfChildData = rawData[childrenMapKey]['child1'][childrenMapKey]['child1'];
+    final Object childOfChildData = rawData[childrenMapKey]['child1'][childrenMapKey]['child1'] as Object;
     expect(childOfChildData, isNotEmpty);
 
     expect(manager.updateScheduled, isTrue);
diff --git a/packages/flutter/test/services/restoration_test.dart b/packages/flutter/test/services/restoration_test.dart
index 3fe2b7522f..1fde21dc0f 100644
--- a/packages/flutter/test/services/restoration_test.dart
+++ b/packages/flutter/test/services/restoration_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 import 'dart:typed_data';
@@ -25,9 +24,9 @@ void main() {
       });
 
       final RestorationManager manager = RestorationManager();
-      final Future<RestorationBucket> rootBucketFuture = manager.rootBucket;
-      RestorationBucket rootBucket;
-      rootBucketFuture.then((RestorationBucket bucket) {
+      final Future<RestorationBucket?> rootBucketFuture = manager.rootBucket;
+      RestorationBucket? rootBucket;
+      rootBucketFuture.then((RestorationBucket? bucket) {
         rootBucket = bucket;
       });
       expect(rootBucketFuture, isNotNull);
@@ -48,14 +47,14 @@ void main() {
       expect(rootBucket, isNotNull);
 
       // Root bucket contains the expected data.
-      expect(rootBucket.read<int>('value1'), 10);
-      expect(rootBucket.read<String>('value2'), 'Hello');
-      final RestorationBucket child = rootBucket.claimChild('child1', debugOwner: null);
+      expect(rootBucket!.read<int>('value1'), 10);
+      expect(rootBucket!.read<String>('value2'), 'Hello');
+      final RestorationBucket child = rootBucket!.claimChild('child1', debugOwner: null);
       expect(child.read<int>('another value'), 22);
 
       // Accessing the root bucket again completes synchronously with same bucket.
-      RestorationBucket synchronousBucket;
-      manager.rootBucket.then((RestorationBucket bucket) {
+      RestorationBucket? synchronousBucket;
+      manager.rootBucket.then((RestorationBucket? bucket) {
         synchronousBucket = bucket;
       });
       expect(synchronousBucket, isNotNull);
@@ -65,16 +64,15 @@ void main() {
     testWidgets('root bucket received from engine before retrieval', (WidgetTester tester) async {
       SystemChannels.restoration.setMethodCallHandler(null);
       final List<MethodCall> callsToEngine = <MethodCall>[];
-      SystemChannels.restoration.setMockMethodCallHandler((MethodCall call) {
+      SystemChannels.restoration.setMockMethodCallHandler((MethodCall call) async {
         callsToEngine.add(call);
-        return null;
       });
       final RestorationManager manager = RestorationManager();
 
       await _pushDataFromEngine(_createEncodedRestorationData1());
 
-      RestorationBucket rootBucket;
-      manager.rootBucket.then((RestorationBucket bucket) => rootBucket = bucket);
+      RestorationBucket? rootBucket;
+      manager.rootBucket.then((RestorationBucket? bucket) => rootBucket = bucket);
       // Root bucket is available synchronously.
       expect(rootBucket, isNotNull);
       // Engine was never asked.
@@ -91,24 +89,24 @@ void main() {
       });
       final RestorationManager manager = RestorationManager();
 
-      RestorationBucket rootBucket;
-      manager.rootBucket.then((RestorationBucket bucket) => rootBucket = bucket);
+      RestorationBucket? rootBucket;
+      manager.rootBucket.then((RestorationBucket? bucket) => rootBucket = bucket);
       expect(rootBucket, isNull);
       expect(callsToEngine.single.method, 'get');
 
       await _pushDataFromEngine(_createEncodedRestorationData1());
       expect(rootBucket, isNotNull);
-      expect(rootBucket.read<int>('value1'), 10);
+      expect(rootBucket!.read<int>('value1'), 10);
 
       result.complete(_createEncodedRestorationData2());
       await tester.pump();
 
-      RestorationBucket rootBucket2;
-      manager.rootBucket.then((RestorationBucket bucket) => rootBucket2 = bucket);
+      RestorationBucket? rootBucket2;
+      manager.rootBucket.then((RestorationBucket? bucket) => rootBucket2 = bucket);
       expect(rootBucket2, isNotNull);
       expect(rootBucket2, same(rootBucket));
-      expect(rootBucket2.read<int>('value1'), 10);
-      expect(rootBucket2.contains('foo'), isFalse);
+      expect(rootBucket2!.read<int>('value1'), 10);
+      expect(rootBucket2!.contains('foo'), isFalse);
     });
 
     testWidgets('root bucket is properly replaced when new data is available', (WidgetTester tester) async {
@@ -116,21 +114,21 @@ void main() {
         return _createEncodedRestorationData1();
       });
       final RestorationManager manager = RestorationManager();
-      RestorationBucket rootBucket;
-      manager.rootBucket.then((RestorationBucket bucket) {
+      RestorationBucket? rootBucket;
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucket = bucket;
       });
       await tester.pump();
       expect(rootBucket, isNotNull);
-      expect(rootBucket.read<int>('value1'), 10);
-      final RestorationBucket child = rootBucket.claimChild('child1', debugOwner: null);
+      expect(rootBucket!.read<int>('value1'), 10);
+      final RestorationBucket child = rootBucket!.claimChild('child1', debugOwner: null);
       expect(child.read<int>('another value'), 22);
 
       bool rootReplaced = false;
-      RestorationBucket newRoot;
+      RestorationBucket? newRoot;
       manager.addListener(() {
         rootReplaced = true;
-        manager.rootBucket.then((RestorationBucket bucket) {
+        manager.rootBucket.then((RestorationBucket? bucket) {
           newRoot = bucket;
         });
         // The new bucket is available synchronously.
@@ -145,9 +143,9 @@ void main() {
 
       child.dispose();
 
-      expect(newRoot.read<int>('foo'), 33);
-      expect(newRoot.read<int>('value1'), null);
-      final RestorationBucket newChild = newRoot.claimChild('childFoo', debugOwner: null);
+      expect(newRoot!.read<int>('foo'), 33);
+      expect(newRoot!.read<int>('value1'), null);
+      final RestorationBucket newChild = newRoot!.claimChild('childFoo', debugOwner: null);
       expect(newChild.read<String>('bar'), 'Hello');
     });
 
@@ -162,9 +160,9 @@ void main() {
       final RestorationManager manager = RestorationManager()..addListener(() {
         listenerCount++;
       });
-      RestorationBucket rootBucket;
+      RestorationBucket? rootBucket;
       bool rootBucketResolved = false;
-      manager.rootBucket.then((RestorationBucket bucket) {
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucketResolved = true;
         rootBucket = bucket;
       });
@@ -180,7 +178,7 @@ void main() {
       // Switch to non-null.
       await _pushDataFromEngine(_createEncodedRestorationData1());
       expect(listenerCount, 1);
-      manager.rootBucket.then((RestorationBucket bucket) {
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucket = bucket;
       });
       expect(rootBucket, isNotNull);
@@ -188,7 +186,7 @@ void main() {
       // Switch to null again.
       await _pushDataFromEngine(_packageRestorationData(enabled: false));
       expect(listenerCount, 2);
-      manager.rootBucket.then((RestorationBucket bucket) {
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucket = bucket;
       });
       expect(rootBucket, isNull);
@@ -203,9 +201,9 @@ void main() {
       });
 
       final RestorationManager manager = RestorationManager();
-      final Future<RestorationBucket> rootBucketFuture = manager.rootBucket;
-      RestorationBucket rootBucket;
-      rootBucketFuture.then((RestorationBucket bucket) {
+      final Future<RestorationBucket?> rootBucketFuture = manager.rootBucket;
+      RestorationBucket? rootBucket;
+      rootBucketFuture.then((RestorationBucket? bucket) {
         rootBucket = bucket;
       });
       result.complete(_createEncodedRestorationData1());
@@ -214,8 +212,8 @@ void main() {
       callsToEngine.clear();
 
       // Schedule a frame.
-      SchedulerBinding.instance.ensureVisualUpdate();
-      rootBucket.write('foo', 1);
+      SchedulerBinding.instance!.ensureVisualUpdate();
+      rootBucket!.write('foo', 1);
       // flushData is no-op because frame is scheduled.
       manager.flushData();
       expect(callsToEngine, isEmpty);
@@ -225,7 +223,7 @@ void main() {
       callsToEngine.clear();
 
       // flushData without frame sends data directly.
-      rootBucket.write('foo', 2);
+      rootBucket!.write('foo', 2);
       manager.flushData();
       expect(callsToEngine, hasLength(1));
     });
@@ -239,40 +237,40 @@ void main() {
       final TestRestorationManager manager = TestRestorationManager();
       expect(manager.isReplacing, isFalse);
 
-      RestorationBucket rootBucket;
-      manager.rootBucket.then((RestorationBucket bucket) {
+      RestorationBucket? rootBucket;
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucket = bucket;
       });
       result.complete(_createEncodedRestorationData1());
       await tester.idle();
       expect(rootBucket, isNotNull);
-      expect(rootBucket.isReplacing, isFalse);
+      expect(rootBucket!.isReplacing, isFalse);
       expect(manager.isReplacing, isFalse);
       tester.binding.scheduleFrame();
       await tester.pump();
       expect(manager.isReplacing, isFalse);
-      expect(rootBucket.isReplacing, isFalse);
+      expect(rootBucket!.isReplacing, isFalse);
 
       manager.receiveDataFromEngine(enabled: true, data: null);
-      RestorationBucket rootBucket2;
-      manager.rootBucket.then((RestorationBucket bucket) {
+      RestorationBucket? rootBucket2;
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucket2 = bucket;
       });
       expect(rootBucket2, isNotNull);
-      expect(rootBucket2, isNot(same(rootBucket)));
+      expect(rootBucket2!, isNot(same(rootBucket)));
       expect(manager.isReplacing, isTrue);
-      expect(rootBucket2.isReplacing, isTrue);
+      expect(rootBucket2!.isReplacing, isTrue);
       await tester.idle();
       expect(manager.isReplacing, isTrue);
-      expect(rootBucket2.isReplacing, isTrue);
+      expect(rootBucket2!.isReplacing, isTrue);
       tester.binding.scheduleFrame();
       await tester.pump();
       expect(manager.isReplacing, isFalse);
-      expect(rootBucket2.isReplacing, isFalse);
+      expect(rootBucket2!.isReplacing, isFalse);
 
       manager.receiveDataFromEngine(enabled: false, data: null);
-      RestorationBucket rootBucket3;
-      manager.rootBucket.then((RestorationBucket bucket) {
+      RestorationBucket? rootBucket3;
+      manager.rootBucket.then((RestorationBucket? bucket) {
         rootBucket3 = bucket;
       });
       expect(rootBucket3, isNull);
@@ -307,7 +305,7 @@ void main() {
 }
 
 Future<void> _pushDataFromEngine(Map<dynamic, dynamic> data) async {
-  await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+  await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
     'flutter/restoration',
     const StandardMethodCodec().encodeMethodCall(MethodCall('push', data)),
     (_) { },
@@ -347,16 +345,16 @@ Map<dynamic, dynamic> _createEncodedRestorationData2() {
   return _packageRestorationData(data: data);
 }
 
-Map<dynamic, dynamic> _packageRestorationData({bool enabled = true, Map<dynamic, dynamic> data}) {
-  final ByteData encoded = const StandardMessageCodec().encodeMessage(data);
+Map<dynamic, dynamic> _packageRestorationData({bool enabled = true, Map<dynamic, dynamic>? data}) {
+  final ByteData? encoded = const StandardMessageCodec().encodeMessage(data);
   return <dynamic, dynamic>{
     'enabled': enabled,
-    'data': encoded == null ? null : encoded.buffer.asUint8List(encoded.offsetInBytes, encoded.lengthInBytes)
+    'data': encoded?.buffer.asUint8List(encoded.offsetInBytes, encoded.lengthInBytes),
   };
 }
 
 class TestRestorationManager extends RestorationManager {
-  void receiveDataFromEngine({@required bool enabled, @required Uint8List data}) {
+  void receiveDataFromEngine({required bool enabled, required Uint8List? data}) {
     handleRestorationUpdateFromEngine(enabled: enabled, data: data);
   }
 }
diff --git a/packages/flutter/test/services/system_chrome_test.dart b/packages/flutter/test/services/system_chrome_test.dart
index d9f115d9cf..1534331870 100644
--- a/packages/flutter/test/services/system_chrome_test.dart
+++ b/packages/flutter/test/services/system_chrome_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:typed_data';
 
@@ -61,11 +60,10 @@ void main() {
   });
 
   test('setApplicationSwitcherDescription missing plugin', () async {
-    final List<ByteData> log = <ByteData>[];
+    final List<ByteData?> log = <ByteData>[];
 
-    ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler('flutter/platform', (ByteData message) {
+    ServicesBinding.instance!.defaultBinaryMessenger.setMockMessageHandler('flutter/platform', (ByteData? message) async {
       log.add(message);
-      return null;
     });
 
     await SystemChrome.setApplicationSwitcherDescription(
diff --git a/packages/flutter/test/services/system_navigator_test.dart b/packages/flutter/test/services/system_navigator_test.dart
index 2219605ff5..bc699856d8 100644
--- a/packages/flutter/test/services/system_navigator_test.dart
+++ b/packages/flutter/test/services/system_navigator_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
diff --git a/packages/flutter/test/services/system_sound_test.dart b/packages/flutter/test/services/system_sound_test.dart
index c16e593a9c..20a6d5adb2 100644
--- a/packages/flutter/test/services/system_sound_test.dart
+++ b/packages/flutter/test/services/system_sound_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
diff --git a/packages/flutter/test/services/text_formatter_test.dart b/packages/flutter/test/services/text_formatter_test.dart
index a0e8ca1e90..b38e332505 100644
--- a/packages/flutter/test/services/text_formatter_test.dart
+++ b/packages/flutter/test/services/text_formatter_test.dart
@@ -2,27 +2,25 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  TextEditingValue testOldValue;
-  TextEditingValue testNewValue;
+  TextEditingValue testOldValue = TextEditingValue.empty;
+  TextEditingValue testNewValue = TextEditingValue.empty;
 
   test('withFunction wraps formatting function', () {
     testOldValue = const TextEditingValue();
     testNewValue = const TextEditingValue();
 
-    TextEditingValue calledOldValue;
-    TextEditingValue calledNewValue;
+    late TextEditingValue calledOldValue;
+    late TextEditingValue calledNewValue;
 
     final TextInputFormatter formatterUnderTest = TextInputFormatter.withFunction(
       (TextEditingValue oldValue, TextEditingValue newValue) {
         calledOldValue = oldValue;
         calledNewValue = newValue;
-        return null;
+        return TextEditingValue.empty;
       }
     );
 
diff --git a/packages/flutter/test/services/text_input_test.dart b/packages/flutter/test/services/text_input_test.dart
index 8e75205e53..8495c9552d 100644
--- a/packages/flutter/test/services/text_input_test.dart
+++ b/packages/flutter/test/services/text_input_test.dart
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:convert' show utf8;
 import 'dart:convert' show jsonDecode;
@@ -15,7 +14,7 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('TextInput message channels', () {
-    FakeTextChannel fakeTextChannel;
+    late FakeTextChannel fakeTextChannel;
 
     setUp(() {
       fakeTextChannel = FakeTextChannel((MethodCall call) async {});
@@ -34,7 +33,7 @@ void main() {
         MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
       ]);
 
-      fakeTextChannel.incoming(const MethodCall('TextInputClient.requestExistingInputState', null));
+      fakeTextChannel.incoming!(const MethodCall('TextInputClient.requestExistingInputState', null));
 
       expect(fakeTextChannel.outgoingCalls.length, 3);
       fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
@@ -47,20 +46,32 @@ void main() {
     });
 
     test('text input client handler responds to reattach with setClient (null TextEditingValue)', () async {
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(TextEditingValue.empty);
       TextInput.attach(client, client.configuration);
       fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
         MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
       ]);
 
-      fakeTextChannel.incoming(const MethodCall('TextInputClient.requestExistingInputState', null));
+      fakeTextChannel.incoming!(const MethodCall('TextInputClient.requestExistingInputState', null));
 
-      expect(fakeTextChannel.outgoingCalls.length, 2);
+      expect(fakeTextChannel.outgoingCalls.length, 3);
       fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
         // From original attach
         MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
-        // From requestExistingInputState
+        // From original attach
         MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+        // From requestExistingInputState
+        const MethodCall(
+            'TextInput.setEditingState',
+            <String, dynamic>{
+              'text': '',
+              'selectionBase': -1,
+              'selectionExtent': -1,
+              'selectionAffinity': 'TextAffinity.downstream',
+              'selectionIsDirectional': false,
+              'composingBase': -1,
+              'composingExtent': -1,
+            }),
       ]);
     });
   });
@@ -162,14 +173,14 @@ void main() {
       expect(client.latestMethodCall, isEmpty);
 
       // Send onConnectionClosed message.
-      final ByteData messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
+      final ByteData? messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[1],
         'method': 'TextInputClient.onConnectionClosed',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'connectionClosed');
@@ -177,14 +188,14 @@ void main() {
 
     test('TextInputClient performPrivateCommand method is called', () async {
       // Assemble a TextInputConnection so we can verify its change in state.
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(TextEditingValue.empty);
       const TextInputConfiguration configuration = TextInputConfiguration();
       TextInput.attach(client, configuration);
 
       expect(client.latestMethodCall, isEmpty);
 
       // Send performPrivateCommand message.
-      final ByteData messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
+      final ByteData? messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[
           1,
           jsonDecode(
@@ -192,10 +203,10 @@ void main() {
         ],
         'method': 'TextInputClient.performPrivateCommand',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'performPrivateCommand');
@@ -204,14 +215,14 @@ void main() {
     test('TextInputClient performPrivateCommand method is called with float',
         () async {
       // Assemble a TextInputConnection so we can verify its change in state.
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(const TextEditingValue());
       const TextInputConfiguration configuration = TextInputConfiguration();
       TextInput.attach(client, configuration);
 
       expect(client.latestMethodCall, isEmpty);
 
       // Send performPrivateCommand message.
-      final ByteData messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
+      final ByteData? messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[
           1,
           jsonDecode(
@@ -219,10 +230,10 @@ void main() {
         ],
         'method': 'TextInputClient.performPrivateCommand',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'performPrivateCommand');
@@ -232,14 +243,14 @@ void main() {
         'TextInputClient performPrivateCommand method is called with CharSequence array',
         () async {
       // Assemble a TextInputConnection so we can verify its change in state.
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(const TextEditingValue());
       const TextInputConfiguration configuration = TextInputConfiguration();
       TextInput.attach(client, configuration);
 
       expect(client.latestMethodCall, isEmpty);
 
       // Send performPrivateCommand message.
-      final ByteData messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
+      final ByteData? messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[
           1,
           jsonDecode(
@@ -247,10 +258,10 @@ void main() {
         ],
         'method': 'TextInputClient.performPrivateCommand',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'performPrivateCommand');
@@ -260,14 +271,14 @@ void main() {
         'TextInputClient performPrivateCommand method is called with CharSequence',
         () async {
       // Assemble a TextInputConnection so we can verify its change in state.
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(const TextEditingValue());
       const TextInputConfiguration configuration = TextInputConfiguration();
       TextInput.attach(client, configuration);
 
       expect(client.latestMethodCall, isEmpty);
 
       // Send performPrivateCommand message.
-      final ByteData messageBytes =
+      final ByteData? messageBytes =
           const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[
           1,
@@ -276,10 +287,10 @@ void main() {
         ],
         'method': 'TextInputClient.performPrivateCommand',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'performPrivateCommand');
@@ -289,14 +300,14 @@ void main() {
         'TextInputClient performPrivateCommand method is called with float array',
         () async {
       // Assemble a TextInputConnection so we can verify its change in state.
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(const TextEditingValue());
       const TextInputConfiguration configuration = TextInputConfiguration();
       TextInput.attach(client, configuration);
 
       expect(client.latestMethodCall, isEmpty);
 
       // Send performPrivateCommand message.
-      final ByteData messageBytes =
+      final ByteData? messageBytes =
           const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[
           1,
@@ -305,10 +316,10 @@ void main() {
         ],
         'method': 'TextInputClient.performPrivateCommand',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'performPrivateCommand');
@@ -317,22 +328,22 @@ void main() {
     test('TextInputClient showAutocorrectionPromptRect method is called',
         () async {
       // Assemble a TextInputConnection so we can verify its change in state.
-      final FakeTextInputClient client = FakeTextInputClient(null);
+      final FakeTextInputClient client = FakeTextInputClient(const TextEditingValue());
       const TextInputConfiguration configuration = TextInputConfiguration();
       TextInput.attach(client, configuration);
 
       expect(client.latestMethodCall, isEmpty);
 
       // Send onConnectionClosed message.
-      final ByteData messageBytes =
+      final ByteData? messageBytes =
           const JSONMessageCodec().encodeMessage(<String, dynamic>{
         'args': <dynamic>[1, 0, 1],
         'method': 'TextInputClient.showAutocorrectionPromptRect',
       });
-      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+      await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage(
         'flutter/textinput',
         messageBytes,
-        (ByteData _) {},
+        (ByteData? _) {},
       );
 
       expect(client.latestMethodCall, 'showAutocorrectionPromptRect');
@@ -376,7 +387,7 @@ class FakeTextInputClient implements TextInputClient {
   TextEditingValue currentTextEditingValue;
 
   @override
-  AutofillScope get currentAutofillScope => null;
+  AutofillScope? get currentAutofillScope => null;
 
   @override
   void performAction(TextInputAction action) {
@@ -415,7 +426,7 @@ class FakeTextChannel implements MethodChannel {
   FakeTextChannel(this.outgoing) : assert(outgoing != null);
 
   Future<dynamic> Function(MethodCall) outgoing;
-  Future<void> Function(MethodCall) incoming;
+  Future<void> Function(MethodCall)? incoming;
 
   List<MethodCall> outgoingCalls = <MethodCall>[];
 
@@ -442,19 +453,17 @@ class FakeTextChannel implements MethodChannel {
   String get name => 'flutter/textinput';
 
   @override
-  void setMethodCallHandler(Future<void> Function(MethodCall call) handler) {
-    incoming = handler;
-  }
+  void setMethodCallHandler(Future<void> Function(MethodCall call)? handler) => incoming = handler;
 
   @override
-  bool checkMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+  bool checkMethodCallHandler(Future<void> Function(MethodCall call)? handler) => throw UnimplementedError();
 
 
   @override
-  void setMockMethodCallHandler(Future<void> Function(MethodCall call) handler)  => throw UnimplementedError();
+  void setMockMethodCallHandler(Future<void> Function(MethodCall call)? handler)  => throw UnimplementedError();
 
   @override
-  bool checkMockMethodCallHandler(Future<void> Function(MethodCall call) handler) => throw UnimplementedError();
+  bool checkMockMethodCallHandler(Future<void> Function(MethodCall call)? handler) => throw UnimplementedError();
 
   void validateOutgoingMethodCalls(List<MethodCall> calls) {
     expect(outgoingCalls.length, calls.length);
@@ -468,8 +477,8 @@ class FakeTextChannel implements MethodChannel {
       if (outgoingString != expectedString) {
         print(
           'Index $i did not match:\n'
-          '  actual:   ${outgoingCalls[i]}\n'
-          '  expected: ${calls[i]}');
+          '  actual:   $outgoingString\n'
+          '  expected: $expectedString');
         hasError = true;
       }
     }