Make popup menus line up to their baseline per the Material spec.

sky/sdk/example/stocks/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/theme/view_configuration.dart';
 
 class StockMenu extends Component {
@@ skipping 18 matching lines @@
     return new Positioned(
       child: new PopupMenu(
         controller: controller,
         items: [
           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
+      right: 0.0,
+      top: 0.0 + kStatusBarHeight
     );
   }
 }