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

samples/searchable_list/web/elements/searchable_list.html

 <!--
 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.
 -->
 
 <polymer-element name="searchable-list">
   <template>
     <div class="container">
       <input class="form-control" type="text" value="{{searchParam}}">
       <template if="{{results.length > 0}}">
         <ul class="list-unstyled">
           <template repeat="{{result in results}}">
             <li>{{result}}</li>
           </template>
         </ul>
       </template>
     </div>
   </template>
 
-  <script type="application/dart;component=1"
-          src="searchable_list.dart"></script>
+  <script type="application/dart" src="searchable_list.dart"></script>
 </polymer-element>