Rename BlockContainer, StackContainer, and FlexContainer

sky/sdk/lib/framework/components2/popup_menu_item.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 '../fn2.dart';
 import 'ink_well.dart';
 
 class PopupMenuItem extends Component {
   PopupMenuItem({ Object key, this.children, this.opacity}) : super(key: key);
 
   final List<UINode> children;
   final double opacity;
 
   UINode build() {
     return new Container(
       constraints: const BoxConstraints(minWidth: 112.0),
       padding: const EdgeDims.all(16.0),
       // TODO(abarth): opacity: opacity,
-      child: new FlexContainer(children: children)
+      child: new Flex(children)
     );
   }
 }