Change test to not extend ByteData

The test `test/widgets/image_resolution_test.dart` has a test class implementing `ByteData`.
Dart will make it a compile-time error to extend/implement/mix-in most of the types of `dart:typed_data` (breaking change: dart-lang/sdk#45115, implementation: https://dart-review.googlesource.com/c/sdk/+/192186), so this test needs to stop doing so.

This is an attempt at a *minimal change* (because I am not familiar with the code at all). It's definitely possible to make other simplicifcations (like inlining the extension).

packages/flutter/test/widgets/image_resolution_test.dart

@@ -14,12 +14,10 @@ import 'package:flutter_test/flutter_test.dart';
 
 import '../image_data.dart';
 
-class TestByteData implements ByteData {
-  TestByteData(this.scale);
-  final double scale;
+ByteData testByteData(double scale) => ByteData(8)..setFloat64(0, scale);
 
-  @override
-  dynamic noSuchMethod(Invocation invocation) => null;
+extension on ByteData {
+  double get scale => this.getFloat64(0);
 }
 
 const String testManifest = '''
@@ -44,22 +42,22 @@ class TestAssetBundle extends CachingAssetBundle {
     late ByteData data;
     switch (key) {
       case 'assets/image.png':
-        data = TestByteData(1.0);
+        data = testByteData(1.0);
         break;
       case 'assets/1.0x/image.png':
-        data = TestByteData(10.0); // see "...with a main asset and a 1.0x asset"
+        data = testByteData(10.0); // see "...with a main asset and a 1.0x asset"
         break;
       case 'assets/1.5x/image.png':
-        data = TestByteData(1.5);
+        data = testByteData(1.5);
         break;
       case 'assets/2.0x/image.png':
-        data = TestByteData(2.0);
+        data = testByteData(2.0);
         break;
       case 'assets/3.0x/image.png':
-        data = TestByteData(3.0);
+        data = testByteData(3.0);
         break;
       case 'assets/4.0x/image.png':
-        data = TestByteData(4.0);
+        data = testByteData(4.0);
         break;
     }
     return SynchronousFuture<ByteData>(data);
@@ -91,7 +89,7 @@ class TestAssetImage extends AssetImage {
   ImageStreamCompleter load(AssetBundleImageKey key, DecoderCallback decode) {
     late ImageInfo imageInfo;
     key.bundle.load(key.name).then<void>((ByteData data) {
-      final TestByteData testData = data as TestByteData;
+      final ByteData testData = data;
       final ui.Image image = images[testData.scale]!;
       assert(image != null, 'Expected ${testData.scale} to have a key in $images');
       imageInfo = ImageInfo(image: image, scale: key.scale);
@@ -312,12 +310,12 @@ void main() {
     Key key = GlobalKey();
     await pumpTreeToLayout(tester, buildImageAtRatio(image, key, ratio, false, images, bundle));
     expect(getRenderImage(tester, key).size, const Size(200.0, 200.0));
-    // Verify we got the 10x scaled image, since the TestByteData said it should be 10x.
+    // Verify we got the 10x scaled image, since the test ByteData said it should be 10x.
     expect(getRenderImage(tester, key).image!.height, 480);
     key = GlobalKey();
     await pumpTreeToLayout(tester, buildImageAtRatio(image, key, ratio, true, images, bundle));
     expect(getRenderImage(tester, key).size, const Size(480.0, 480.0));
-    // Verify we got the 10x scaled image, since the TestByteData said it should be 10x.
+    // Verify we got the 10x scaled image, since the test ByteData said it should be 10x.
     expect(getRenderImage(tester, key).image!.height, 480);
   });