Added TodoMVC startup benchmarks.

samples/third_party/todomvc_performance/web/performance.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library todomvc_performance;
+
+import 'dart:async';
+import 'dart:html';
+import 'dart:js' as js;
+import 'package:polymer/polymer.dart';
+import 'package:web_components/polyfill.dart';
+import 'elements/td_model.dart';
+
+/**
+ * This test determines how fast the TodoMVC app has loaded.
+ */
+main() {
+  initPolymer();
+  Polymer.onReady.then((_) {
+    var endInitTime = new DateTime.now();
+    window.postMessage(endInitTime.millisecondsSinceEpoch, '*');
+  });
+}