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

pkg/polymer/test/template_distribute_dynamic_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:async';
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
@@ skipping 34 matching lines @@
       if (window.ShadowDOMPolyfill) {
         var actualChildren = this.$.echo.impl.children;
         chai.assert.equal(actualChildren.length, 4,
           'shadowDOMPolyfill distributes expected number of actual children.');
       }
       */
     });
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('inserted called', () => (querySelector('x-test') as XTest).onTestDone);
-}
+});