Add a menu to the stocks app

sky/examples/widgets-fn/widgets_app.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 '../../framework/fn.dart';
 import '../../framework/components/button.dart';
 import '../../framework/components/popup_menu.dart';
-import '../../framework/components/popup_menu_item.dart';
 
 class WidgetsApp extends App {
   static final Style _menuStyle = new Style('''
     position: absolute;
     top: 200px;
     left: 200px;''');
 
   Node build() {
     return new Container(
       children: [
         new Button(key: 'Go', content: new Text('Go'), level: 1),
         new Button(key: 'Back', content: new Text('Back'), level: 3),
         new Container(
           style: _menuStyle,
           children: [
             new PopupMenu(
-              children: [
-                  new PopupMenuItem(key: '1', children: [new Text('People & options')]),
-                  new PopupMenuItem(key: '2', children: [new Text('New group conversation')]),
-                  new PopupMenuItem(key: '3', children: [new Text('Turn history off')]),
-                  new PopupMenuItem(key: '4', children: [new Text('Archive')]),
-                  new PopupMenuItem(key: '5', children: [new Text('Delete')]),
-                  new PopupMenuItem(key: '6', children: [new Text('Un-merge SMS')]),
-                  new PopupMenuItem(key: '7', children: [new Text('Help & feeback')]),
+              items: [
+                [new Text('People & options')],
+                [new Text('New group conversation')],
+                [new Text('Turn history off')],
+                [new Text('Archive')],
+                [new Text('Delete')],
+                [new Text('Un-merge SMS')],
+                [new Text('Help & feeback')],
               ],
               level: 4),
             ]
           )
         ]
       );
   }
 }