Let Dart code running in Sky add events to the trace timeline

sky/framework/fn.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.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
 
+final sky.Tracing _tracing = sky.window.tracing;
+
 bool _initIsInCheckedMode() {
   String testFn(i) { double d = i; return d.toString(); }
   try {
     testFn('not a double');
   } catch (ex) {
     return true;
   }
   return false;
 }
 
@@ skipping 684 matching lines @@
     _unmountedComponents.clear();
   } finally {
     _notifingMountStatus = false;
   }
 }
 
 List<Component> _dirtyComponents = new List<Component>();
 bool _buildScheduled = false;
 bool _inRenderDirtyComponents = false;
 
+void _buildDirtyComponents() {
+  _tracing.begin('fn::_buildDirtyComponents');
 
-void _buildDirtyComponents() {
   Stopwatch sw;
   if (_shouldLogRenderDuration)
     sw = new Stopwatch()..start();
 
   try {
     _inRenderDirtyComponents = true;
 
     _dirtyComponents.sort((a, b) => a._order - b._order);
     for (var comp in _dirtyComponents) {
       comp._buildIfDirty();
     }
 
     _dirtyComponents.clear();
     _buildScheduled = false;
   } finally {
     _inRenderDirtyComponents = false;
   }
 
   _notifyMountStatusChanged();
 
   if (_shouldLogRenderDuration) {
     sw.stop();
-    print("Render took ${sw.elapsedMicroseconds} microseconds");
+    print('Render took ${sw.elapsedMicroseconds} microseconds');
   }
+
+  _tracing.end('fn::_buildDirtyComponents');
 }
 
 void _scheduleComponentForRender(Component c) {
   assert(!_inRenderDirtyComponents);
   _dirtyComponents.add(c);
 
   if (!_buildScheduled) {
     _buildScheduled = true;
     new Future.microtask(_buildDirtyComponents);
   }
@@ skipping 142 matching lines @@
 
 abstract class App extends Component {
   sky.Node _host;
 
   App() : super(stateful: true) {
     _host = sky.document.createElement('div');
     sky.document.appendChild(_host);
     _scheduleComponentForRender(this);
   }
 }