[rfw] Increase tolerance for material widget tests (#7148)

This PR introduces a tolerance for rfw material widget tests to fix the golden test error in flutter/flutter#151611.

Part of a fix for flutter/flutter#151659.

packages/rfw/test/material_widgets_test.dart

@@ -9,6 +9,8 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:rfw/formats.dart' show parseLibraryFile;
 import 'package:rfw/rfw.dart';
 
+import 'tolerant_comparator.dart'
+    if (dart.library.js_interop) 'tolerant_comparator_web.dart';
 import 'utils.dart';
 
 void main() {
@@ -22,6 +24,13 @@ void main() {
       ..update(materialName, createMaterialWidgets());
   }
 
+  setUpAll(() {
+    setUpTolerantComparator(
+      testPath: 'test/material_widget_test.dart',
+      precisionTolerance: 0.00002,
+    );
+  });
+
   testWidgets('Material widgets', (WidgetTester tester) async {
     final Runtime runtime = setupRuntime();
     final DynamicContent data = DynamicContent();

packages/rfw/test/tolerant_comparator.dart

@@ -0,0 +1,56 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+/// Sets [_TolerantGoldenFileComparator] as the default golden file comparator
+/// in tests.
+void setUpTolerantComparator(
+    {required String testPath, required double precisionTolerance}) {
+  final GoldenFileComparator oldComparator = goldenFileComparator;
+  final _TolerantGoldenFileComparator newComparator =
+      _TolerantGoldenFileComparator(Uri.parse(testPath),
+          precisionTolerance: precisionTolerance);
+
+  goldenFileComparator = newComparator;
+
+  addTearDown(() => goldenFileComparator = oldComparator);
+}
+
+class _TolerantGoldenFileComparator extends LocalFileComparator {
+  _TolerantGoldenFileComparator(
+    super.testFile, {
+    required double precisionTolerance,
+  })  : assert(
+          0 <= precisionTolerance && precisionTolerance <= 1,
+          'precisionTolerance must be between 0 and 1',
+        ),
+        _precisionTolerance = precisionTolerance;
+
+  /// How much the golden image can differ from the test image.
+  ///
+  /// It is expected to be between 0 and 1. Where 0 is no difference (the same image)
+  /// and 1 is the maximum difference (completely different images).
+  final double _precisionTolerance;
+
+  @override
+  Future<bool> compare(Uint8List imageBytes, Uri golden) async {
+    final ComparisonResult result = await GoldenFileComparator.compareLists(
+      imageBytes,
+      await getGoldenBytes(golden),
+    );
+
+    final bool passed =
+        result.passed || result.diffPercent <= _precisionTolerance;
+    if (passed) {
+      result.dispose();
+      return true;
+    }
+
+    final String error = await generateFailureOutput(result, golden, basedir);
+    result.dispose();
+    throw FlutterError(error);
+  }
+}

packages/rfw/test/tolerant_comparator_web.dart

@@ -0,0 +1,6 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+void setUpTolerantComparator(
+    {required String testPath, required double precisionTolerance}) {}