Revert "Make flutter frontend server null safe" (flutter/engine#36417)

engine/src/flutter/flutter_frontend_server/bin/starter.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.8
 library frontend_server;
 
 import 'dart:io';

engine/src/flutter/flutter_frontend_server/lib/server.dart

@@ -4,6 +4,7 @@
 
 // ignore_for_file: avoid_print
 
+// @dart=2.8
 library flutter_frontend_server;
 
 import 'dart:async';
@@ -25,9 +26,9 @@ import 'package:path/path.dart' as path;
 /// version for testing.
 Future<int> starter(
   List<String> args, {
-  frontend.CompilerInterface? compiler,
-  Stream<List<int>>? input,
-  StringSink? output,
+  frontend.CompilerInterface compiler,
+  Stream<List<int>> input,
+  StringSink output,
 }) async {
   ArgResults options;
   try {

engine/src/flutter/flutter_frontend_server/pubspec.yaml

@@ -18,7 +18,7 @@ homepage: https://flutter.dev
 # relative to this directory into //third_party/dart
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.2.2 <3.0.0"
 
 dependencies:
   args: any