Make tracing work in the new .dart world.

R=abarth@chromium.org, viktorl@google.com

Review URL: https://codereview.chromium.org/1152483006

engine/bindings/scripts/templates/dart_blink.template

@@ -19,6 +19,8 @@ part '{{callback.name}}.dart';
 Window window;
 Document get document => window.document;
 
+final Tracing tracing = new Tracing();
+
 View view;
 
 typedef EventListener(Event event);

engine/core/frame/Tracing.idl

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-interface Tracing {
+[
+    Constructor()
+] interface Tracing {
   void begin(DOMString name);
   void end(DOMString name);
 };

examples/raw/spinning_square.dart

@@ -7,6 +7,7 @@ import 'dart:sky';
 double timeBase = null;
 
 void beginFrame(double timeStamp) {
+  tracing.begin('beginFrame');
   if (timeBase == null)
     timeBase = timeStamp;
   double delta = timeStamp - timeBase;
@@ -17,6 +18,7 @@ void beginFrame(double timeStamp) {
                   new Paint()..setARGB(255, 0, 255, 0));
   view.picture = canvas.endRecording();
   view.scheduleFrame();
+  tracing.end('beginFrame');
 }
 
 void main() {