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

pkg/polymer/test/instance_attrs_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:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/polymer.dart';
 
 // Note: we use @CustomTag to make this type reflectable.
 @CustomTag('my-element')
 class MyElement extends PolymerElement {
   MyElement.created() : super.created();
 
   // This is here so that [attributes] can be read via mirrors in polymer
   // expressions (@CustomTag in this class makes the attribute reflectable).
   get attributes => super.attributes;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('attributes were deserialized', () {
     var elem = querySelector('my-element');
 
     expect(elem.attributes, {'foo': '123', 'bar': 'hi', 'baz': 'world'},
         reason: 'attributes should be copied to instance');
 
     var text = elem.shadowRoot.text;
     // Collapse adjacent whitespace like a browser would:
     text = text.replaceAll('\n', ' ').replaceAll(new RegExp(r'\s+'), ' ');
 
     expect(text, " foo: 123 bar: hi baz: world ",
         reason: 'text should match expected HTML template');
   });
-}
+});