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

pkg/polymer/example/scoped_style/my_test.html

 <polymer-element name="my-test">
   <template>
     <style>
       p { color: red;}
     </style>
     <p>Inside element, should be red</p>
   </template>
-  <script type="application/dart;component=1">
+  <script type="application/dart">
     import 'package:polymer/polymer.dart';
 
     @CustomTag('my-test')
     class MyTest extends PolymerElement {}
   </script>
 </polymer-element>