Bring back initPolymer, allow boot.js only if using "polymer_experimental.html".

samples/third_party/todomvc/web/lib-elements/polymer_localstorage.html

 <!--
 Copyright 2013 The Polymer Authors. All rights reserved.
 Use of this source code is governed by a BSD-style
 license that can be found in the LICENSE file.
 -->
 <!--
 /**
  * polymer-localstorage provides access to localStorage.
  *
  * Example:
  *
  *     <polymer-localstorage name="my-app-storage" value="{{value}}">
  *     </polymer-localstorage>
  */
 -->
 <polymer-element name="polymer-localstorage">
   <template>
     <style>
       @host {
         * {
           display: none;
         }
       }
     </style>
   </template>
-  <script type="application/dart;component=1"
-          src="polymer_localstorage.dart"></script>
+  <script type="application/dart" src="polymer_localstorage.dart"></script>
 </polymer-element>