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

pkg/polymer/test/property_change_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.property_change_test;
 
 import 'dart:async';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
@@ skipping 27 matching lines @@
     bar = 'bar';
     new Future(() { zonk = 'zonk'; });
   }
 
   barChanged() {
     expect(bar, 'bar', reason: 'change in ready calls *Changed');
     _bar.complete();
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('bar change detected', () => _bar.future);
   test('zonk change detected', () => _zonk.future);
-}
+});