Make tracing work in the new .dart world.

sky/examples/raw/spinning_square.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:sky';
 
 double timeBase = null;
 
 void beginFrame(double timeStamp) {
+  tracing.begin('beginFrame');
   if (timeBase == null)
     timeBase = timeStamp;
   double delta = timeStamp - timeBase;
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotateDegrees(delta / 10);
   canvas.drawRect(new Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
                   new Paint()..setARGB(255, 0, 255, 0));
   view.picture = canvas.endRecording();
   view.scheduleFrame();
+  tracing.end('beginFrame');
 }
 
 void main() {
   view.setBeginFrameCallback(beginFrame);
   view.scheduleFrame();
 }