Expose the diff from ComparisonResult - #77014

await expectLater(find.byType(MyTestWidget),
    matchesGoldenFile('my_test_widget.png'));

class GoldenDiffComparator extends LocalFileComparator {
  GoldenDiffComparator(String testFile, {double tolerance = 0.005,}) : super(Uri.parse(testFile));

  @override
  Future<bool> compare(Uint8List imageBytes, Uri golden) async {
    final ComparisonResult result = await GoldenFileComparator.compareLists(
      imageBytes,
      await getGoldenBytes(golden),
    );

    if (!result.passed && result.diffPercent > _kGoldenDiffTolerance) {
      final String error = await generateFailureOutput(result, golden, basedir);
      throw FlutterError(error);
    }
    if (!result.passed) {
      log('A tolerable difference of ${result.diffPercent * 100}% was found when comparing $golden.');
    }
    return result.passed || result.diffPercent <= _kGoldenDiffTolerance;
  }
}

goldenFileComparator = GoldenDiffComparator(path.join(
  (goldenFileComparator as LocalFileComparator).basedir.toString(),
  goldenFileKey,
));

expectLater(actual, matchesGoldenFile(goldenPath), reason: reason, skip: skip || !Platform.isLinux);