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

pkg/polymer/test/entered_view_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.entered_view_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 11 matching lines @@
   int enteredCount = 0;
 
   XInner.created() : super.created();
 
   enteredView() {
     enteredCount++;
     super.enteredView();
   }
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
   Polymer.register('x-inner', XInner);
   Polymer.register('x-outer', XOuter);
 
   setUp(() => Polymer.onReady);
 
   test('element created properly', () {
     XOuter outer = querySelector('x-outer');
     outer.expand = true;
     return outer.onMutation(outer.shadowRoot).then((_) {
       // Element upgrade is also using mutation observers. Wait another tick so
       // it goes before we do.
       return new Future(() {
         XInner inner = outer.shadowRoot.querySelector('x-inner');
         expect(inner.enteredCount, 1, reason: 'enteredView should be called');
       });
     });
   });
-}
+});