Migrate some more non-test utils for tests (#67351)

packages/flutter/test/material/data_table_test_utils.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
-
 class Dessert {
   Dessert(this.name, this.calories, this.fat, this.carbs, this.protein, this.sodium, this.calcium, this.iron);
 

packages/flutter/test/material/feedback_tester.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';
 
 /// Tracks how often feedback has been requested since its instantiation.

packages/flutter/test/painting/image_test_utils.dart

@@ -10,7 +10,6 @@ import 'package:flutter/painting.dart';
 
 import 'package:flutter_test/flutter_test.dart';
 
-
 class TestImageProvider extends ImageProvider<TestImageProvider> {
   TestImageProvider(this.testImage);
 

packages/flutter/test/services/message_codecs_testing.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 = 2.8
-
 import 'dart:typed_data';
 
 import 'package:flutter/services.dart';
 import '../flutter_test_alternative.dart';
 
 void checkEncoding<T>(MessageCodec<T> codec, T message, List<int> expectedBytes) {
-  final ByteData encoded = codec.encodeMessage(message);
+  final ByteData encoded = codec.encodeMessage(message)!;
   expect(
     encoded.buffer.asUint8List(0, encoded.lengthInBytes),
     orderedEquals(expectedBytes),
@@ -18,17 +16,17 @@ void checkEncoding<T>(MessageCodec<T> codec, T message, List<int> expectedBytes)
 }
 
 void checkEncodeDecode<T>(MessageCodec<T> codec, T message) {
-  final ByteData encoded = codec.encodeMessage(message);
+  final ByteData? encoded = codec.encodeMessage(message);
   final T decoded = codec.decodeMessage(encoded);
   if (message == null) {
     expect(encoded, isNull);
     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(),
-      orderedEquals(encoded.buffer.asUint8List()),
+      orderedEquals(encoded!.buffer.asUint8List()),
     );
   }
 }