Add a menu to the stocks app

sky/framework/components/popup_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 '../fn.dart';
 import '../theme/colors.dart';
 import 'material.dart';
+import 'popup_menu_item.dart';
 
 class PopupMenu extends Component {
   static final Style _style = new Style('''
     border-radius: 2px;
+    padding: 8px 0;
     background-color: ${Grey[50]};'''
   );
 
-  List<Node> children;
+  List<List<Node>> items;
   int level;
 
-  PopupMenu({ Object key, this.children, this.level }) : super(key: key);
+  PopupMenu({ Object key, this.items, this.level }) : super(key: key);
 
   Node build() {
+    List<Node> children = [];
+    int i = 0;
+    items.forEach((List<Node> item) {
+      children.add(new PopupMenuItem(key: i++, children: item));
+    });
+
     return new Material(
       style: _style,
       children: children,
       level: level
     );
   }
 }