Wire up popup menus to back button

sky/sdk/example/stocks/lib/stock_menu.dart

 // Copyright 2015 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 'dart:sky' as sky;
 
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/checkbox.dart';
+import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/popup_menu_item.dart';
 import 'package:sky/widgets/popup_menu.dart';
 
 export 'package:sky/widgets/popup_menu.dart' show PopupMenuStatus;
 
 class StockMenu extends Component {
   StockMenu({
     String key,
     this.showing,
     this.onStatusChanged,
+    this.navigator,
     this.autorefresh: false,
     this.onAutorefreshChanged
   }) : super(key: key);
 
-  bool showing;
-  PopupMenuStatusChangedCallback onStatusChanged;
+  final bool showing;
+  final PopupMenuStatusChangedCallback onStatusChanged;
+  final Navigator navigator;
   final bool autorefresh;
   final ValueChanged onAutorefreshChanged;
 
   Widget build() {
     var checkbox = new Checkbox(
       value: this.autorefresh,
       onChanged: this.onAutorefreshChanged
     );
 
     return new Positioned(
       child: new PopupMenu(
         items: [
           new PopupMenuItem(child: new Text('Add stock')),
           new PopupMenuItem(child: new Text('Remove stock')),
           new PopupMenuItem(child: new Flex([new Flexible(child: new Text('Autorefresh')), checkbox])),
         ],
         level: 4,
         showing: showing,
-        onStatusChanged: onStatusChanged
+        onStatusChanged: onStatusChanged,
+        navigator: navigator
       ),
       right: sky.view.paddingRight,
       top: sky.view.paddingTop
     );
   }
 }