Rename UINode to Widget.

sky/sdk/lib/widgets/button_base.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 'basic.dart';
 
 abstract class ButtonBase extends Component {
 
   ButtonBase({ String key, this.highlight: false }) : super(key: key);
 
@@ skipping 12 matching lines @@
     setState(() {
       highlight = false;
     });
   }
   void _handlePointerCancel(_) {
     setState(() {
       highlight = false;
     });
   }
 
-  UINode build() {
+  Widget build() {
     return new EventListenerNode(
       buildContent(),
       onPointerDown: _handlePointerDown,
       onPointerUp: _handlePointerUp,
       onPointerCancel: _handlePointerCancel
     );
   }
 
-  UINode buildContent();
+  Widget buildContent();
 
 }