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

pkg/polymer/test/js_interop_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.
 
 library polymer.test.web.js_interop_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:js';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
 
 @CustomTag("dart-element")
 class DartElement extends PolymerElement {
   DartElement.created() : super.created();
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('dart-element upgraded', () {
     expect(querySelector('dart-element') is DartElement, true,
         reason: 'dart-element upgraded');
   });
 
   test('js-element in body', () => testInterop(
       querySelector('js-element')));
 
   test('js-element in dart-element', () => testInterop(
       querySelector('dart-element').shadowRoot.querySelector('js-element')));
-}
+});
 
 testInterop(jsElem) {
   expect(jsElem.shadowRoot.text, 'FOOBAR');
   var interop = new JsObject.fromBrowserObject(jsElem);
   expect(interop['baz'], 42, reason: 'can read JS custom element properties');
 
   jsElem.attributes['baz'] = '123';
   return flush().then((_) {
     expect(interop['baz'], 123, reason: 'attribute reflected to property');
     expect(jsElem.shadowRoot.text, 'FOOBAR', reason: 'text unchanged');
@@ skipping 17 matching lines @@
 /// Calls Platform.flush() to flush Polymer.js pending operations, e.g.
 /// dirty checking for data-bindings.
 Future flush() {
   var Platform = context['Platform'];
   Platform.callMethod('flush');
 
   var completer = new Completer();
   Platform.callMethod('endOfMicrotask', [() => completer.complete()]);
   return completer.future;
 }