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

pkg/polymer/test/property_observe_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 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 63 matching lines @@
 
   @ObserveProperty('noogle')
   validate() => super.validate();
 
   ready() {
     super.ready();
     noogle = 'noogle';
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('changes detected', () => _done.future);
-}
+});