Re-work MaterialComponent

sky/framework/components/checkbox.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 'button_base.dart';
 
 typedef void ValueChanged(value);
 
 class Checkbox extends ButtonBase {
@@ skipping 52 matching lines @@
   }
 
   void _handleClick(sky.Event e) {
     onChanged(!checked);
   }
 
   Node build() {
     return new Container(
       style: _style,
       children: [
-        super.build(),
         new Container(
           style: highlight ? _containerHighlightStyle : _containerStyle,
           children: [
             new Container(
               style: checked ? _checkedStyle : _uncheckedStyle
             )
           ]
         )
       ]
     )
   }
 }