Improve the openning animation for PopupMenu

sky/examples/stocks-fn/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 'package:sky/framework/components/popup_menu.dart';
 import 'package:sky/framework/fn.dart';
 import 'package:sky/framework/theme/view-configuration.dart';
-import 'stock_arrow.dart';
-import 'stock_data.dart';
 
 class StockMenu extends Component {
   static final Style _style = new Style('''
     position: absolute;
     right: 8px;
     top: ${8 + kStatusBarHeight}px;''');
 
-  StockMenu({Object key}) : super(key: key);
+  PopupMenuController controller;
+
+  StockMenu({Object key, this.controller}) : super(key: key);
 
   Node build() {
     return new Container(
       style: _style,
       children: [
         new PopupMenu(
+          controller: controller,
           items: [
             [new Text('Add stock')],
             [new Text('Remove stock')],
             [new Text('Help & feeback')],
           ],
           level: 4)
       ]
     );
   }
 }