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

pkg/polymer/test/take_attributes_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('x-foo')
@@ skipping 31 matching lines @@
   XArray.created() : super.created();
   @observable List values;
 }
 
 @CustomTag('x-obj')
 class XObj extends PolymerElement {
   XObj.created() : super.created();
   @observable var values = {};
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('take attributes', () {
     queryXTag(x) => document.querySelector(x);
 
     expect(queryXTag("#foo0").boolean, true);
     expect(queryXTag("#foo1").boolean, false);
     expect(queryXTag("#foo2").boolean, true);
@@ skipping 48 matching lines @@
     // Array deserialization tests
     expect(queryXTag("#arr1").values, [0, 1, 2]);
     expect(queryXTag("#arr2").values, [33]);
     // Object deserialization tests
     expect(queryXTag("#obj1").values, { 'name': 'Brandon',
         'nums': [1, 22, 33] });
     expect(queryXTag("#obj2").values, { "color": "Red" });
     expect(queryXTag("#obj3").values, { 'movie': 'Buckaroo Banzai',
         'DOB': '07/31/1978' });
   });
-}
+});