Enable setting the UINode key for ParentDataNodes

sky/examples/stocks2/lib/stock_app.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/components2/tool_bar.dart';
 import 'package:sky/framework/components2/drawer.dart';
 import 'package:sky/framework/components2/drawer_header.dart';
 import 'package:sky/framework/components2/floating_action_button.dart';
 import 'package:sky/framework/components2/icon.dart';
 import 'package:sky/framework/components2/icon_button.dart';
@@ skipping 112 matching lines @@
         new MenuItem(
           key: 'Account Balance',
           icon: 'action/account_balance',
           children: [new Text('Account Balance')]),
         new MenuDivider(key: 'div1'),
         new MenuItem(
           key: 'Optimistic Menu Item',
           icon: 'action/thumb_up',
           onGestureTap: (event) => _handleStockModeChange(StockMode.optimistic),
           children: [
-            new FlexExpandingChild(new Text('Optimistic'), 1),
+            new FlexExpandingChild(new Text('Optimistic'), flex: 1),
             new Radio(key: 'optimistic-radio', value: StockMode.optimistic, groupValue: _stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuItem(
           key: 'Pessimistic Menu Item',
           icon: 'action/thumb_down',
           onGestureTap: (event) => _handleStockModeChange(StockMode.pessimistic),
           children: [
-            new FlexExpandingChild(new Text('Pessimistic'), 1),
+            new FlexExpandingChild(new Text('Pessimistic'), flex: 1),
             new Radio(key: 'pessimistic-radio', value: StockMode.pessimistic, groupValue: _stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuDivider(key: 'div2'),
         new MenuItem(
           key: 'Settings',
           icon: 'action/settings',
           children: [new Text('Settings')]),
         new MenuItem(
           key: 'Help & Feedback',
           icon: 'action/help',
@@ skipping 63 matching lines @@
 }
 
 void main() {
   print("starting stocks app!");
   App app = new StocksApp();
   app.appView.onFrame = () {
     // uncomment this for debugging:
     // app.appView.debugDumpRenderTree();
   };
 }