Add BlockContainer to fn2

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';
 
 class ContainerApp extends App {
   UINode build() {
     return new EventListenerNode(
-      new Rectangle(0xFF00FFFF),
+      new BlockContainer(children: [
+        new Rectangle(0xFF00FFFF, key: 1),
+        new Rectangle(0xFF00FF00, key: 2),
+        new Rectangle(0xFF0000FF, key: 3)
+      ]),
       onPointerDown: _handlePointerDown);
   }
 
   void _handlePointerDown(sky.PointerEvent event) {
     print("_handlePointerDown");
   }
 }
 
 void main() {
   new ContainerApp();
 }