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

pkg/polymer/test/nested_binding_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.nested_binding_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 10 matching lines @@
   _runTest(_) {
     expect($['fruit'].text.trim(), 'Short name: [pears]');
     _testDone.complete();
   }
 
   ready() {
     onMutation($['fruit']).then(_runTest);
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('ready called',
       () => (querySelector('my-test') as MyTest)._testDone.future);
-}
+});