Add a simple golden-file test to package:integration_test. (#159233)

Closes https://github.com/flutter/flutter/issues/72045.

Let's see if I wired this correctly and if `Flutter Gold` knows to check
based on the files changed.

packages/integration_test/pubspec.yaml

@@ -38,6 +38,10 @@ dependencies:
   vector_math: 2.1.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   webdriver: 3.0.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
 
+dev_dependencies:
+  flutter_goldens:
+    sdk: flutter
+
 flutter:
   plugin:
     platforms:

packages/integration_test/test/_goldens_io.dart

@@ -0,0 +1,5 @@
+// Copyright 2014 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.
+
+export 'package:flutter_goldens/flutter_goldens.dart' show testExecutable;

packages/integration_test/test/_goldens_web.dart

@@ -0,0 +1,8 @@
+// Copyright 2014 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 'dart:async';
+
+// package:flutter_goldens is not used as part of the test process for web.
+Future<void> testExecutable(FutureOr<void> Function() testMain, {String? namePrefix}) async => testMain();

packages/integration_test/test/flutter_test_config.dart

@@ -0,0 +1,12 @@
+// Copyright 2014 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 'dart:async';
+
+import '_goldens_io.dart' if (dart.library.js_interop) '_goldens_web.dart' as flutter_goldens;
+
+Future<void> testExecutable(FutureOr<void> Function() testMain) {
+  // Enable golden file testing using Skia Gold.
+  return flutter_goldens.testExecutable(testMain, namePrefix: 'integration');
+}

packages/integration_test/test/golden_screenshot_test.dart

@@ -0,0 +1,23 @@
+// Copyright 2014 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.
+
+@Tags(<String>['reduced-test-set'])
+library;
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+
+  testWidgets('a screenshot of a Placeholder', (WidgetTester tester) async {
+    await tester.pumpWidget(const Placeholder());
+
+    await expectLater(
+      find.byType(Placeholder),
+      matchesGoldenFile('placeholder.png'),
+    );
+  });
+}