Re-work MaterialComponent

sky/framework/components/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 '../fn.dart';
-import 'material.dart';
 
-abstract class ButtonBase extends Material {
+abstract class ButtonBase extends Component {
   bool highlight = false;
 
   ButtonBase({ Object key }) : super(key: key) {
     events.listen('pointerdown', _handlePointerDown);
     events.listen('pointerup', _handlePointerUp);
     events.listen('pointercancel', _handlePointerCancel);
   }
 
   void _handlePointerDown(_) {
     setState(() {
       highlight = true;
     });
   }
   void _handlePointerUp(_) {
     setState(() {
       highlight = false;
     });
   }
   void _handlePointerCancel(_) {
     setState(() {
       highlight = false;
     });
   }
 }