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

pkg/polymer/test/register_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';
 
 @CustomTag('x-html')
@@ skipping 22 matching lines @@
   XPolymerElement.created() : super.created();
 }
 
 /// Dart-specific test:
 /// This element is registered from code without an associated polymer-element.
 class XButtonElement extends ButtonElement with Polymer, Observable {
   XButtonElement.created() : super.created();
 }
 
 
-@initMethod
 main() {
+  initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('elements upgraded', () {
     expect(querySelector('x-html') is XHtmlElement, isTrue);
     expect(querySelector('x-html-two') is XHtml2Element, isTrue);
     expect(querySelector('#x-div') is XDivElement, isTrue);
     expect(querySelector('#x-div-two') is XDiv2Element, isTrue);
   });
@@ skipping 17 matching lines @@
     test('type extension', () {
       Polymer.registerSync('x-button', XButtonElement, extendsTag: 'button');
 
       expect(document.createElement('button', 'x-button') is XButtonElement,
           isTrue, reason: 'should have been registered');
       expect(document.querySelector('[is=x-button]') is XButtonElement, isTrue,
           reason: 'elements on page should be upgraded');
     });
   });
 }