Fix blend test diffrate on mac (flutter#17198)

* Fix error message for golden failure. Fix blend test diffrate when running on MacOS

* Revert MacOS adjustment

lib/web_ui/test/golden_tests/engine/canvas_blend_golden_test.dart

@@ -19,7 +19,8 @@ void main() async {
 
   // Commit a recording canvas to a bitmap, and compare with the expected
   Future<void> _checkScreenshot(RecordingCanvas rc, String fileName,
-      {Rect region = const Rect.fromLTWH(0, 0, 500, 500)}) async {
+      {Rect region = const Rect.fromLTWH(0, 0, 500, 500),
+       double maxDiffRatePercent = 0.0}) async {
     final EngineCanvas engineCanvas = BitmapCanvas(screenRect);
 
     rc.apply(engineCanvas);
@@ -29,7 +30,7 @@ void main() async {
     try {
       sceneElement.append(engineCanvas.rootElement);
       html.document.body.append(sceneElement);
-      await matchGoldenFile('$fileName.png', region: region, maxDiffRatePercent: 0.0);
+      await matchGoldenFile('$fileName.png', region: region, maxDiffRatePercent: maxDiffRatePercent);
     } finally {
       // The page is reused across tests, so remove the element after taking the
       // Scuba screenshot.
@@ -76,7 +77,8 @@ void main() async {
           ..style = PaintingStyle.fill
           ..color = const Color.fromARGB(128, 255, 0, 0));
     rc.restore();
-    await _checkScreenshot(rc, 'canvas_blend_circle_diff_color');
+    await _checkScreenshot(rc, 'canvas_blend_circle_diff_color',
+        maxDiffRatePercent: operatingSystem == OperatingSystem.macOs ? 2.95 : 0);
   });
 
   test('Blend circle and text with multiply', () async {
@@ -112,7 +114,8 @@ void main() async {
     rc.drawImage(createTestImage(), Offset(135.0, 130.0),
         Paint()..blendMode = BlendMode.multiply);
     rc.restore();
-    await _checkScreenshot(rc, 'canvas_blend_image_multiply');
+    await _checkScreenshot(rc, 'canvas_blend_image_multiply',
+        maxDiffRatePercent: operatingSystem == OperatingSystem.macOs ? 2.95 : 0);
   });
 }