Make PopupMenuItem take a single child instead of an implicitly-flex list of children.

sky/examples/stocks2/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/widgets/checkbox.dart';
 import 'package:sky/widgets/popup_menu.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
@@ skipping 12 matching lines @@
   UINode build() {
     var checkbox = new Checkbox(
       value: this.autorefresh,
       onChanged: this.onAutorefreshChanged
     );
 
     return new Positioned(
       child: new PopupMenu(
         controller: controller,
         items: [
-          [new Text('Add stock')],
-          [new Text('Remove stock')],
-          [new Flexible(child: new Text('Autorefresh')), checkbox],
+          new Text('Add stock'),
+          new Text('Remove stock'),
+          new Flex([new Flexible(child: new Text('Autorefresh')), checkbox]),
         ],
         level: 4
       ),
       right: 8.0,
       top: 8.0 + kStatusBarHeight
     );
   }
 }