Add a menu to the stocks app

sky/framework/components/popup_menu.dart

Index: sky/framework/components/popup_menu.dart
diff --git a/sky/framework/components/popup_menu.dart b/sky/framework/components/popup_menu.dart
index 4a4aca66f97e85456ba5ae946b02369b1403a06e..8ed4e6c0b4e10967d85b885424371792b8740c9a 100644
--- a/sky/framework/components/popup_menu.dart
+++ b/sky/framework/components/popup_menu.dart
@@ -5,19 +5,27 @@
 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,