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

sky/framework/components/fixed_height_scrollable.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 '../animation/scroll_behavior.dart';
 import '../fn.dart';
+import '../tracing.dart';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:async';
 import 'scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
   static final Style _style = new Style('''
     overflow: hidden;
     position: relative;
     will-change: transform;'''
   );
 
   static final Style _scrollAreaStyle = new Style('''
     position:relative;
     will-change: transform;'''
   );
 
   double _height = 0.0;
   double _itemHeight;
 
   FixedHeightScrollable({
     Object key,
     ScrollBehavior scrollBehavior
   }) : super(key: key, scrollBehavior: scrollBehavior);
 
   void _measureHeights() {
-    if (_itemHeight != null)
-      return;
-    var root = getRoot();
-    if (root == null)
-      return;
-    var item = root.firstChild.firstChild;
-    if (item == null)
-      return;
-    sky.ClientRect scrollRect = root.getBoundingClientRect();
-    sky.ClientRect itemRect = item.getBoundingClientRect();
-    assert(scrollRect.height > 0);
-    assert(itemRect.height > 0);
+    trace('FixedHeightScrollable::_measureHeights', () {
+      if (_itemHeight != null)
+        return;
+      var root = getRoot();
+      if (root == null)
+        return;
+      var item = root.firstChild.firstChild;
+      if (item == null)
+        return;
+      sky.ClientRect scrollRect = root.getBoundingClientRect();
+      sky.ClientRect itemRect = item.getBoundingClientRect();
+      assert(scrollRect.height > 0);
+      assert(itemRect.height > 0);
 
-    setState(() {
-      _height = scrollRect.height;
-      _itemHeight = itemRect.height;
+      setState(() {
+        _height = scrollRect.height;
+        _itemHeight = itemRect.height;
+      });
     });
   }
 
   Node buildContent() {
     var itemNumber = 0;
     var drawCount = 1;
     var transformStyle = '';
 
     if (_itemHeight == null)
       new Future.microtask(_measureHeights);
@@ skipping 24 matching lines @@
       children: [
         new Container(
           style: _scrollAreaStyle,
           inlineStyle: transformStyle,
           children: buildItems(itemNumber, drawCount)
         )
       ]
     );
   }
 }