Make decodeImageFromList mockable (#25864)

packages/flutter/lib/src/painting/image_decoder.dart

@@ -4,15 +4,24 @@
 
 import 'dart:async';
 import 'dart:typed_data';
-import 'dart:ui' as ui show Image, decodeImageFromList;
+import 'dart:ui' as ui show Codec, FrameInfo, Image;
+
+import 'binding.dart';
 
 /// Creates an image from a list of bytes.
 ///
 /// This function attempts to interpret the given bytes an image. If successful,
 /// the returned [Future] resolves to the decoded image. Otherwise, the [Future]
 /// resolves to null.
-Future<ui.Image> decodeImageFromList(Uint8List list) {
-  final Completer<ui.Image> completer = Completer<ui.Image>();
-  ui.decodeImageFromList(list, completer.complete);
-  return completer.future;
+///
+/// If the image is animated, this returns the first frame. Consider
+/// [instantiateImageCodec] if support for animated images is necessary.
+///
+/// This function differs from [ui.decodeImageFromList] in that it defers to
+/// [PaintingBinding.instantiateImageCodec], and therefore can be mocked in
+/// tests.
+Future<ui.Image> decodeImageFromList(Uint8List bytes) async {
+  final ui.Codec codec = await PaintingBinding.instance.instantiateImageCodec(bytes);
+  final ui.FrameInfo frameInfo = await codec.getNextFrame();
+  return frameInfo.image;
 }

packages/flutter/test/painting/image_decoder_test.dart

@@ -6,15 +6,19 @@ import 'dart:typed_data';
 import 'dart:ui' as ui;
 
 import 'package:flutter/painting.dart';
-import '../flutter_test_alternative.dart';
 
+import '../flutter_test_alternative.dart';
+import 'binding_test.dart';
 import 'image_data.dart';
 
 void main() {
+  final PaintingBindingSpy binding = PaintingBindingSpy();
   test('Image decoder control test', () async {
+    expect(binding.instantiateImageCodecCalledCount, 0);
     final ui.Image image = await decodeImageFromList(Uint8List.fromList(kTransparentImage));
     expect(image, isNotNull);
     expect(image.width, 1);
     expect(image.height, 1);
+    expect(binding.instantiateImageCodecCalledCount, 1);
   });
 }

packages/flutter/test/widgets/box_decoration_test.dart

@@ -36,6 +36,7 @@ class TestImageProvider extends ImageProvider<TestImageProvider> {
 }
 
 Future<void> main() async {
+  AutomatedTestWidgetsFlutterBinding();
   TestImageProvider.image = await decodeImageFromList(Uint8List.fromList(kTransparentImage));
 
   testWidgets('DecoratedBox handles loading images', (WidgetTester tester) async {

packages/flutter/test/widgets/shape_decoration_test.dart

@@ -14,6 +14,7 @@ import '../rendering/mock_canvas.dart';
 import 'test_border.dart' show TestBorder;
 
 Future<void> main() async {
+  AutomatedTestWidgetsFlutterBinding();
   final ui.Image rawImage = await decodeImageFromList(Uint8List.fromList(kTransparentImage));
   final ImageProvider image = TestImageProvider(0, 0, image: rawImage);
   testWidgets('ShapeDecoration.image', (WidgetTester tester) async {