Add support for transforms to container

sky/examples/fn2/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 'dart:sky' as sky;
 import 'package:sky/framework/fn2.dart';
 import 'package:sky/framework/rendering/box.dart';
 
 class ContainerApp extends App {
   UINode build() {
     return new EventListenerNode(
       new BlockContainer(children: [
-        new Rectangle(0xFF00FFFF, key: 1),
         new Container(
           padding: new EdgeDims.all(10.0),
           margin: new EdgeDims.all(10.0),
           desiredSize: new sky.Size(double.INFINITY, 100.0),
           decoration: new BoxDecoration(backgroundColor: 0xFF00FF00),
           child: new BlockContainer(
               children: [
                   new Container(
                       decoration: new BoxDecoration(backgroundColor: 0xFFFFFF00),
                       desiredSize: new sky.Size(double.INFINITY, 20.0)
                   )
               ])),
-        new Rectangle(0xFF0000FF, key: 3)
       ]),
       onPointerDown: _handlePointerDown);
   }
 
   void _handlePointerDown(sky.PointerEvent event) {
     print("_handlePointerDown");
   }
 }
 
 void main() {
   new ContainerApp();
 }