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

pkg/polymer/example/component/news/web/news-component.html

 <polymer-element name="x-news" extends="ul">
   <template>
     <style scoped>
       div.breaking {
           color: Red;
           font-size: 20px;
           border: 1px dashed Purple;
       }
       div.other {
           padding: 2px 0 0 0;
           border: 1px solid Cyan;
       }
     </style>
     <div class="breaking">
       <h2>Breaking Stories</h2>
       <ul>
           <content select=".breaking"></content>
       </ul>
     </div>
     <div class="other">
       <h2>Other News</h2>
       <ul>
           <content></content>
       </ul>
     </div>
   </template>
-  <script type="application/dart;component=1">
+  <script type="application/dart">
     import 'dart:html';
     import 'package:polymer/polymer.dart';
 
     class XNews extends UListElement with Polymer {
       XNews.created() : super.created() {
         polymerCreated();
       }
     }
 
     @initMethod
     _init() {
       Polymer.register('x-news', XNews);
     }
   </script>
 </polymer-element>