Remove onFrame callback

sky/sdk/example/widgets/block_viewport.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/base/scheduler.dart' as scheduler;
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/block_viewport.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/scaffold.dart';
 import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';
 import 'package:sky/widgets/widget.dart';
 
@@ skipping 107 matching lines @@
       onGestureTap: (_) {
         removeBox(index);
       }
     );
   }
 
 }
 
 void main() {
   runApp(new BlockViewportApp());
-  SkyBinding.instance.onFrame = () {
-    // uncomment this for debugging:
-    // SkyBinding.instance.debugDumpRenderTree();
-  };
+  // scheduler.addPersistentFrameCallback((_) {
+  //   SkyBinding.instance.debugDumpRenderTree();
+  // });
 }