Add golden tests for Opacity with offset (#24253)

See https://github.com/flutter/flutter/issues/23890

bin/internal/goldens.version

-8c478bbaf27447f3d612959705b305e7d1293526
+e07cc0cb4fdf912062e71a6fd97cc91478d6e3b9

packages/flutter/lib/src/rendering/layer.dart

@@ -1142,6 +1142,7 @@ class OpacityLayer extends ContainerLayer {
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(IntProperty('alpha', alpha));
+    properties.add(DiagnosticsProperty<Offset>('offset', offset));
   }
 }
 

packages/flutter/test/widgets/opacity_test.dart

@@ -2,8 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io';
+
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
 
 import '../rendering/mock_canvas.dart';
 import 'semantics_tester.dart';
@@ -147,4 +150,37 @@ void main() {
 
     semantics.dispose();
   });
+
+  testWidgets('offset is correctly handled in Opacity', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: SingleChildScrollView(
+            child: RepaintBoundary(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.stretch,
+                children: List<Widget>.generate(10, (int index) {
+                  return Opacity(
+                    opacity: 0.5,
+                    child: Padding(
+                      padding: const EdgeInsets.all(5.0),
+                      child: Container(
+                          color: Colors.blue,
+                          height: 50
+                      ),
+                    )
+                  );
+                }),
+              ),
+            )
+          )
+        )
+      )
+    );
+    await expectLater(
+      find.byType(RepaintBoundary).first,
+      matchesGoldenFile('opacity_test.offset.1.png'),
+      skip: !Platform.isLinux,
+    );
+  });
 }