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

pkg/polymer/test/attr_mustache_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:async';
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:template_binding/template_binding.dart';
@@ skipping 26 matching lines @@
   }
 }
 
 @CustomTag('x-test')
 class XTest extends PolymerElement {
   XTest.created() : super.created();
 
   @observable var src = 'testSource';
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('mustache attributes', () {
     final xtest = document.querySelector('#test');
     final xtarget = xtest.shadowRoot.querySelector('#target');
     return xtarget.foundSrc;
   });
-}
+});