Remove onFrame callback

sky/sdk/example/widgets/sector.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 'dart:math' as math;
 
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/widgets/basic.dart';
@@ skipping 128 matching lines @@
           ),
           body: buildBody()
         )
       )
     );
   }
 }
 
 void main() {
   runApp(new SectorApp());
-  SkyBinding.instance.onFrame = () {
-    // uncomment this for debugging:
-    // SkyBinding.instance.debugDumpRenderTree();
-  };
 }