Improve the openning animation for PopupMenu

sky/examples/stocks-fn/lib/stock_menu.dart

Index: sky/examples/stocks-fn/lib/stock_menu.dart
diff --git a/sky/examples/stocks-fn/lib/stock_menu.dart b/sky/examples/stocks-fn/lib/stock_menu.dart
index 1f680992103ab7e99f174cbe955f8b33bca5fff1..65db00d3d93fc1f1ad9f14960716445fc1633671 100644
--- a/sky/examples/stocks-fn/lib/stock_menu.dart
+++ b/sky/examples/stocks-fn/lib/stock_menu.dart
@@ -5,8 +5,6 @@
 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('''
@@ -14,13 +12,16 @@ class StockMenu extends Component {
     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')],