Introduce boot.js: this finally makes it possible to load and run Todomvc

example/todomvc/web/app.dart

 // Copyright (c) 2013, 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 app;
 
 import 'dart:html';
 import 'package:observe/observe.dart';
 import 'package:polymer/polymer.dart';
 import 'model.dart';
@@ skipping 10 matching lines @@
   void addTodo(Event e) {
     e.preventDefault(); // don't submit the form
     var input = getShadowRoot('todo-app').query('#new-todo');
     if (input.value == '') return;
     app.todos.add(new Todo(input.value));
     input.value = '';
   }
 
   void clear() => app.clearDone();
 }
+
+_init() {
+  registerPolymerElement('todo-app', () => new TodoApp());
+}