Remove one more use of mirrors: Components now have to explicitly sync their fields.

sky/sdk/lib/framework/components2/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 'package:sky/framework/theme2/colors.dart' as colors;
 
 import 'dart:sky' as sky;
 import '../fn2.dart';
 import '../rendering/box.dart';
 import '../rendering/object.dart';
 import 'button_base.dart';
 
 typedef void ValueChanged(value);
 
 class Checkbox extends ButtonBase {
 
   Checkbox({ Object key, this.onChanged, this.checked }) : super(key: key);
 
   bool checked;
   ValueChanged onChanged;
 
+  void syncFields(Checkbox source) {
+    checked = source.checked;
+    onChanged = source.onChanged;
+    super.syncFields(source);
+  }
+
   void _handleClick(sky.Event e) {
     onChanged(!checked);
   }
 
   UINode buildContent() {
     // TODO(jackson): This should change colors with the theme
     sky.Color color = highlight ? colors.Purple[500] : const sky.Color(0x8A000000);
     const double edgeSize = 20.0;
     const double edgeRadius = 1.0;
     return new EventListenerNode(
@@ skipping 26 matching lines @@
               canvas.drawPath(path, paint);
             }
           }
         )
       ),
       onGestureTap: _handleClick
     );
   }
 
 }