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

sky/framework/tracing.dart

Index: sky/framework/tracing.dart
diff --git a/sky/framework/tracing.dart b/sky/framework/tracing.dart
new file mode 100644
index 0000000000000000000000000000000000000000..557bd2818841b95f55ce76fda55e5348d8e53836
--- /dev/null
+++ b/sky/framework/tracing.dart
@@ -0,0 +1,12 @@
+// 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' as sky;
+
+dynamic trace(String name, Function f) {
+  sky.window.tracing.begin(name);
+  var result = f();
+  sky.window.tracing.end(name);
+  return result;
+}