Delay time dilation so users see the checkbox (#7803)

Fixes https://github.com/flutter/flutter/issues/5066

examples/flutter_gallery/lib/gallery/app.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:flutter/foundation.dart' show defaultTargetPlatform;
 import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart' show timeDilation;
@@ -51,8 +53,23 @@ class GalleryAppState extends State<GalleryApp> {
   bool _useLightTheme = true;
   bool _showPerformanceOverlay = false;
   bool _checkerboardRasterCacheImages = false;
+  double _timeDilation = 1.0;
+  TargetPlatform _platform = defaultTargetPlatform;
+
+  Timer _timeDilationTimer;
 
-  TargetPlatform platform = defaultTargetPlatform;
+  @override
+  void initState() {
+    _timeDilation = timeDilation;
+    super.initState();
+  }
+
+  @override
+  void dispose() {
+    _timeDilationTimer?.cancel();
+    _timeDilationTimer = null;
+    super.dispose();
+  }
 
   @override
   Widget build(BuildContext context) {
@@ -77,13 +94,25 @@ class GalleryAppState extends State<GalleryApp> {
       } : null,
       onPlatformChanged: (TargetPlatform value) {
         setState(() {
-          platform = value;
+          _platform = value;
         });
       },
-      timeDilation: timeDilation,
+      timeDilation: _timeDilation,
       onTimeDilationChanged: (double value) {
         setState(() {
-          timeDilation = value;
+          _timeDilationTimer?.cancel();
+          _timeDilationTimer = null;
+          _timeDilation = value;
+          if (_timeDilation > 1.0) {
+            // We delay the time dilation change long enough that the user can see
+            // that the checkbox in the drawer has started reacting, then we slam
+            // on the brakes so that they see that the time is in fact now dilated.
+            _timeDilationTimer = new Timer(const Duration(milliseconds: 150), () {
+              timeDilation = _timeDilation;
+            });
+          } else {
+            timeDilation = _timeDilation;
+          }
         });
       },
       onSendFeedback: config.onSendFeedback,
@@ -99,7 +128,7 @@ class GalleryAppState extends State<GalleryApp> {
     return new MaterialApp(
       title: 'Flutter Gallery',
       color: Colors.grey[500],
-      theme: (_useLightTheme ? _kGalleryLightTheme : _kGalleryDarkTheme).copyWith(platform: platform),
+      theme: (_useLightTheme ? _kGalleryLightTheme : _kGalleryDarkTheme).copyWith(platform: _platform),
       showPerformanceOverlay: _showPerformanceOverlay,
       checkerboardRasterCacheImages: _checkerboardRasterCacheImages,
       routes: _kRoutes,