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

pkg/polymer/test/prop_attr_bind_reflection_test.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.
 
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('my-child-element')
@@ skipping 11 matching lines @@
   observeAttributeProperty(name) { }
 }
 
 @CustomTag('my-element')
 class MyElement extends PolymerElement {
   @observable int volume = 11;
 
   MyElement.created() : super.created();
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('attribute reflected to property name', () {
     var child = querySelector('my-element')
         .shadowRoot.querySelector('my-child-element');
     expect(child.lowercase, 11);
     expect(child.camelCase, 11);
 
     expect('11', child.attributes['lowercase']);
     expect('11', child.attributes['camelcase']);
   });
-}
+});