Extend Sky's RenderFlex with intrinsic sizes and compute cross-size height

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/framework/fn2.dart';
 import 'package:sky/framework/components2/popup_menu.dart';
 import 'package:sky/framework/components2/checkbox.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
   PopupMenuController controller;
 
   StockMenu({Object key, this.controller, this.autorefresh: false, this.onAutorefreshChanged}) : super(key: key);
 
   final bool autorefresh;
   final ValueChanged onAutorefreshChanged;
 
   UINode build() {
     var checkbox = new Checkbox(
       checked: this.autorefresh,
       onChanged: this.onAutorefreshChanged
     );
 
     return new StackPositionedChild(
       new PopupMenu(
         controller: controller,
         items: [
           [new Text('Add stock')],
           [new Text('Remove stock')],
-          // [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
+          [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
         ],
         level: 4
       ),
       right: 8.0,
       top: 8.0 + kStatusBarHeight
     );
   }
 }