Made some pixel tests fuzzy (flutter#154680)

This is a forward fix for the failures seen in https://github.com/flutter/engine/pull/54714/checks?check_run_id=29742599916 on flutter/engine#54714

issue: flutter#127855

Author: gaaclarke

packages/flutter/test/painting/decoration_test.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:ui' as ui show ColorFilter, Image;
+import 'dart:ui' as ui show Color, ColorFilter, Image;
 
 import 'package:fake_async/fake_async.dart';
 import 'package:flutter/foundation.dart';
@@ -14,6 +14,36 @@ import '../image_data.dart';
 import '../painting/mocks_for_image_cache.dart';
 import '../rendering/rendering_tester.dart';
 
+/// Positive result if the colors would be mapped to the same argb8888 color.
+class _ColorMatcher extends Matcher {
+  _ColorMatcher(this._target);
+
+  final ui.Color _target;
+
+  @override
+  Description describe(Description description) {
+    return description.add('matches "$_target"');
+  }
+
+  @override
+  bool matches(dynamic item, Map<dynamic, dynamic> matchState) {
+    if (item is ui.Color) {
+      return item.colorSpace == _target.colorSpace &&
+          (item.a - _target.a).abs() <= (1 / 255) &&
+          (item.r - _target.r).abs() <= (1 / 255) &&
+          (item.g - _target.g).abs() <= (1 / 255) &&
+          (item.b - _target.b).abs() <= (1 / 255);
+    } else {
+      return false;
+    }
+  }
+
+}
+
+Matcher _matchesColor(ui.Color color) {
+  return _ColorMatcher(color);
+}
+
 class TestCanvas implements Canvas {
   final List<Invocation> invocations = <Invocation>[];
 
@@ -326,7 +356,7 @@ void main() {
     expect(call.positionalArguments[3], isA<Paint>());
     final Paint paint = call.positionalArguments[3] as Paint;
     expect(paint.colorFilter, colorFilter);
-    expect(paint.color, const Color(0x7F000000)); // 0.5 opacity
+    expect(paint.color, _matchesColor(const Color(0x7F000000))); // 0.5 opacity
     expect(paint.filterQuality, FilterQuality.high);
     expect(paint.isAntiAlias, true);
     expect(paint.invertColors, isTrue);