first draft of polymer element tutorial

src/site/docs/tutorials/polymer-intro/examples/stopwatch/build.dart

+#!/usr/bin/env dart
+// Copyright (c) 2012, 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.
+
+import 'package:polymer/builder.dart';
+
+void main() {
+  build(entryPoints: ['web/index.html']); // lint and optionally build
+}