Make it possible to detect PopupMenuItem presses

sky/sdk/example/stocks/lib/stock_menu.dart

Index: sky/sdk/example/stocks/lib/stock_menu.dart
diff --git a/sky/sdk/example/stocks/lib/stock_menu.dart b/sky/sdk/example/stocks/lib/stock_menu.dart
index c122695666b8ac2bd2ea8b9286452c9aaf0067aa..6ef3f4e8138afa0403b5413441fd738b0eb5e935 100644
--- a/sky/sdk/example/stocks/lib/stock_menu.dart
+++ b/sky/sdk/example/stocks/lib/stock_menu.dart
@@ -4,9 +4,10 @@
 
 import 'dart:sky' as sky;
 
+import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/checkbox.dart';
+import 'package:sky/widgets/popup_menu_item.dart';
 import 'package:sky/widgets/popup_menu.dart';
-import 'package:sky/widgets/basic.dart';
 
 export 'package:sky/widgets/popup_menu.dart' show PopupMenuStatus;
 
@@ -33,9 +34,9 @@ class StockMenu extends Component {
     return new Positioned(
       child: new PopupMenu(
         items: [
-          new Text('Add stock'),
-          new Text('Remove stock'),
-          new Flex([new Flexible(child: new Text('Autorefresh')), checkbox]),
+          new PopupMenuItem(child: new Text('Add stock')),

[Hixie, 2015/07/14 17:57:54]

should we have a PopupMenuItem.text('Add stock') constructor?

+          new PopupMenuItem(child: new Text('Remove stock')),
+          new PopupMenuItem(child: new Flex([new Flexible(child: new Text('Autorefresh')), checkbox])),
         ],
         level: 4,
         showing: showing,