Add a scene diffSnapshot.

packages/devtools_app/lib/src/screens/memory/panes/diff/diff_pane.dart

@@ -68,23 +68,35 @@ class _SnapshotItemContent extends StatelessWidget {
           );
         }
 
-        return Column(
-          children: [
-            const SizedBox(height: denseRowSpacing),
-            SnapshotControlPane(controller: controller),
-            const SizedBox(height: denseRowSpacing),
-            Expanded(
-              child: SnapshotView(
-                controller: controller,
-              ),
-            ),
-          ],
-        );
+        return SnapshotInstanceItemPane(controller: controller);
       },
     );
   }
 }
 
+@visibleForTesting
+class SnapshotInstanceItemPane extends StatelessWidget {
+  const SnapshotInstanceItemPane({super.key, required this.controller});
+
+  final DiffPaneController controller;
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: [
+        const SizedBox(height: denseRowSpacing),
+        SnapshotControlPane(controller: controller),
+        const SizedBox(height: denseRowSpacing),
+        Expanded(
+          child: SnapshotView(
+            controller: controller,
+          ),
+        ),
+      ],
+    );
+  }
+}
+
 /// `\v` adds vertical space
 const _snapshotDocumentation = '''
 Take a **heap snapshot** to view current memory allocation:

packages/devtools_app/test/scenes/memory/diff_snapshot.dart

@@ -0,0 +1,86 @@
+// Copyright 2022 The Chromium 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:devtools_app/src/config_specific/ide_theme/ide_theme.dart';
+import 'package:devtools_app/src/config_specific/import_export/import_export.dart';
+import 'package:devtools_app/src/primitives/feature_flags.dart';
+import 'package:devtools_app/src/screens/memory/memory_controller.dart';
+import 'package:devtools_app/src/screens/memory/panes/diff/controller/diff_pane_controller.dart';
+import 'package:devtools_app/src/screens/memory/panes/diff/diff_pane.dart';
+import 'package:devtools_app/src/screens/memory/shared/heap/model.dart';
+import 'package:devtools_app/src/service/service_manager.dart';
+import 'package:devtools_app/src/shared/globals.dart';
+import 'package:devtools_app/src/shared/notifications.dart';
+import 'package:devtools_test/devtools_test.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:stager/stager.dart';
+
+import '../../test_data/memory/heap/heap_data.dart';
+
+/// To run:
+/// flutter run -t test/scenes/memory/diff_snapshot.stager_app.dart -d macos
+class DiffSnapshotScene extends Scene {
+  late DiffPaneController diffController;
+  late MemoryController controller;
+  late FakeServiceManager fakeServiceManager;
+
+  @override
+  Widget build() {
+    return Scaffold(
+      body: SnapshotInstanceItemPane(controller: diffController),
+    );
+  }
+
+  @override
+  Future<void> setUp() async {
+    FeatureFlags.memoryDiffing = true;
+
+    setGlobal(OfflineModeController, OfflineModeController());
+    setGlobal(IdeTheme, IdeTheme());
+    setGlobal(NotificationService, NotificationService());
+
+    fakeServiceManager =
+        FakeServiceManager(service: FakeServiceManager.createFakeService());
+    mockConnectedApp(
+      fakeServiceManager.connectedApp!,
+      isFlutterApp: true,
+      isProfileBuild: true,
+      isWebApp: false,
+    );
+    setGlobal(ServiceConnectionManager, fakeServiceManager);
+
+    controller = MemoryController(
+      diffPaneController: diffController =
+          DiffPaneController(_TestSnapshotTaker()),
+    );
+
+    await diffController.takeSnapshot();
+    await diffController.takeSnapshot();
+  }
+
+  @override
+  String get title => '$DiffSnapshotScene';
+
+  void tearDown() {
+    FeatureFlags.memoryDiffing = false;
+  }
+}
+
+/// Provides test snapshots.
+class _TestSnapshotTaker implements SnapshotTaker {
+  bool firstTime = true;
+  int _nextIndex = 0;
+
+  @override
+  Future<AdaptedHeapData?> take() async {
+    // This delay is needed for UI to start showing the progress indicator.
+    await Future.delayed(const Duration(milliseconds: 100));
+    final result = await goldenHeapTests[_nextIndex].loadHeap();
+
+    _nextIndex = (_nextIndex + 1) % goldenHeapTests.length;
+
+    return result;
+  }
+}