Migrate gallery test to null safety (#84065)

dev/integration_tests/flutter_gallery/test_driver/scroll_perf_test.dart

@@ -2,14 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:flutter_driver/flutter_driver.dart';
 import 'package:test/test.dart' hide TypeMatcher, isInstanceOf;
 
 void main() {
   group('scrolling performance test', () {
-    FlutterDriver driver;
+    late FlutterDriver driver;
 
     setUpAll(() async {
       driver = await FlutterDriver.connect();
@@ -18,7 +16,6 @@ void main() {
     });
 
     tearDownAll(() async {
-      if (driver != null)
         driver.close();
     });
 

dev/integration_tests/flutter_gallery/test_driver/scroll_perf_web.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 = 2.9
-
 import 'package:flutter/material.dart';
 import 'package:flutter_driver/driver_extension.dart';
 import 'package:flutter_gallery/gallery/app.dart' show GalleryApp;

dev/integration_tests/flutter_gallery/test_driver/scroll_perf_web_test.dart

@@ -2,21 +2,18 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:flutter_driver/flutter_driver.dart';
 import 'package:test/test.dart' hide TypeMatcher, isInstanceOf;
 
 void main() {
   group('scrolling performance test', () {
-    FlutterDriver driver;
+    late FlutterDriver driver;
 
     setUpAll(() async {
       driver = await FlutterDriver.connect();
     });
 
     tearDownAll(() async {
-      if (driver != null)
         driver.close();
     });
 

dev/integration_tests/flutter_gallery/test_driver/transitions_perf_e2e.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 = 2.9
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_gallery/demo_lists.dart';

dev/integration_tests/flutter_gallery/test_driver/transitions_perf_e2e_test.dart

@@ -2,15 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:integration_test/integration_test_driver.dart' as driver;
 
 Future<void> main() => driver.integrationDriver(
   timeout: const Duration(minutes: 5),
-  responseDataCallback: (Map<String, dynamic> data) async {
+  responseDataCallback: (Map<String, dynamic>? data) async {
     await driver.writeResponseData(
-      data['performance'] as Map<String, dynamic>,
+      data!['performance'] as Map<String, dynamic>,
       testOutputFilename: 'e2e_perf_summary',
     );
   }

dev/integration_tests/flutter_gallery/test_driver/transitions_perf_e2e_with_semantics.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 = 2.9
 import 'transitions_perf_e2e.dart' as transitions_perf;
 
 void main() {

dev/integration_tests/flutter_gallery/test_driver/transitions_perf_hybrid_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 = 2.9
-
 import 'transitions_perf_test.dart' as transitions_perf_test;
 
 void main([List<String> args = const <String>[]]) => transitions_perf_test.main(

dev/integration_tests/flutter_gallery/test_driver/transitions_perf_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 = 2.9
-
 import 'dart:convert' show JsonEncoder, json;
 
 import 'package:file/file.dart';
@@ -27,8 +25,8 @@ List<String> _allDemos = <String>[];
 /// it into a histogram, and saves to a JSON file.
 Future<void> saveDurationsHistogram(List<Map<String, dynamic>> events, String outputPath) async {
   final Map<String, List<int>> durations = <String, List<int>>{};
-  Map<String, dynamic> startEvent;
-  int frameStart;
+  Map<String, dynamic>? startEvent;
+  int? frameStart;
 
   // Save the duration of the first frame after each 'Start Transition' event.
   for (final Map<String, dynamic> event in events) {
@@ -46,7 +44,7 @@ Future<void> saveDurationsHistogram(List<Map<String, dynamic>> events, String ou
         assert(phase == 'E');
         final String routeName = startEvent['args']['to'] as String;
         durations[routeName] ??= <int>[];
-        durations[routeName].add(timestamp - frameStart);
+        durations[routeName]!.add(timestamp - frameStart!);
         startEvent = null;
         frameStart = null;
       }
@@ -105,7 +103,7 @@ Future<void> saveDurationsHistogram(List<Map<String, dynamic>> events, String ou
 /// home screen twice.
 Future<void> runDemos(List<String> demos, FlutterDriver driver) async {
   final SerializableFinder demoList = find.byValueKey('GalleryDemoList');
-  String currentDemoCategory;
+  String? currentDemoCategory;
 
   for (final String demo in demos) {
     if (kSkippedDemos.contains(demo))
@@ -158,7 +156,7 @@ void main([List<String> args = const <String>[]]) {
   final bool withSemantics = args.contains('--with_semantics');
   final bool hybrid = args.contains('--hybrid');
   group('flutter gallery transitions', () {
-    FlutterDriver driver;
+    late FlutterDriver driver;
     setUpAll(() async {
       driver = await FlutterDriver.connect();
 
@@ -176,7 +174,6 @@ void main([List<String> args = const <String>[]]) {
     });
 
     tearDownAll(() async {
-      if (driver != null)
         await driver.close();
     });
 

dev/integration_tests/flutter_gallery/test_driver/transitions_perf_with_semantics_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 = 2.9
 import 'transitions_perf_test.dart' as transitions_perf_test;
 
 void main() {