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

pkg/polymer/example/component/news/test/news_index_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:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 /// This test runs the news example and checks the state of the initial page.
-@initMethod
 main() {
+  initPolymer();
   useHtmlConfiguration();
 
   extractLinks(nodes) => nodes.where((n) => n is Element)
       .map((n) => n.query('a').href.split('/').last).toList();
 
   test('initial state', () {
     final listComp = querySelector('ul');
     final items = listComp.querySelectorAll('li');
     expect(items.length, 6);
     expect(extractLinks(items), ['1', '2', '3', '4', '4', '5']);
     expect(listComp is Polymer, true, reason: 'x-news should be created');
 
     final contents = listComp.shadowRoot.querySelectorAll('content');
     expect(contents.length, 2, reason: 'news has 2 content tags');
     expect(extractLinks(contents[0].getDistributedNodes()),
         ['3', '5'], reason: 'breaking stories first');
     expect(extractLinks(contents[1].getDistributedNodes()),
         ['1', '2', '4', '4'], reason: 'other stories after breaking stories');
   });
 }