Give ParentDataNodes snappier names

sky/examples/widgets/container.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/rendering/box.dart';
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/basic.dart';
 
 class ContainerApp extends App {
@@ skipping 20 matching lines @@
               onPressed: () => print("Hello World")
             ),
             new RaisedButton(
               key: 'b2',
               child: new Text('DISABLED'),
               onPressed: () => print("Hello World"),
               enabled: false
             )
           ])
         ),
-        new FlexExpandingChild(
-          new Container(
+        new Flexible(
+          child: new Container(
             decoration: new BoxDecoration(backgroundColor: const Color(0xFF00FFFF))
           )
         ),
       ],
       direction: FlexDirection.vertical,
       justifyContent: FlexJustifyContent.spaceBetween
     );
   }
 }
 
 void main() {
   new ContainerApp();
 }