Make it possible to detect PopupMenuItem presses

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/popup_menu_item.dart';
 import 'package:sky/widgets/popup_menu.dart';
-import 'package:sky/widgets/basic.dart';
 
 export 'package:sky/widgets/popup_menu.dart' show PopupMenuStatus;
 
 class StockMenu extends Component {
   StockMenu({
     String key,
     this.showing,
     this.onStatusChanged,
     this.autorefresh: false,
     this.onAutorefreshChanged
   }) : super(key: key);
 
   bool showing;
   PopupMenuStatusChangedCallback onStatusChanged;
   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 Text('Add stock'),
-          new Text('Remove stock'),
-          new Flex([new Flexible(child: new Text('Autorefresh')), checkbox]),
+          new PopupMenuItem(child: new Text('Add stock')),

[Hixie, 2015/07/14 17:57:54]

should we have a PopupMenuItem.text('Add stock') constructor?

+          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
       ),
       right: sky.view.paddingRight,
       top: sky.view.paddingTop
     );
   }
 }