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

pkg/polymer/test/attr_deserialize_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';
 
 @CustomTag('my-element')
 class MyElement extends PolymerElement {
   MyElement.created() : super.created();
 
   @published double volume;
   @published int factor;
   @published bool crankIt;
   @published String msg;
   @published DateTime time;
   @published Object json;
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('attributes were deserialized', () {
     MyElement elem = querySelector('my-element');
     final msg = 'property should match attribute.';
     expect(elem.volume, 11.0, reason: '"volume" should match attribute');
     expect(elem.factor, 3, reason: '"factor" should match attribute');
     expect(elem.crankIt, true, reason: '"crankIt" should match attribute');
     expect(elem.msg, "Yo!", reason: '"msg" should match attribute');
     expect(elem.time, DateTime.parse('2013-08-08T18:34Z'),
         reason: '"time" should match attribute');
     expect(elem.json, {'here': 'is', 'some': 'json', 'x': 123},
         reason: '"json" should match attribute');
 
     var text = elem.shadowRoot.text;
     // Collapse adjacent whitespace like a browser would:
     text = text.replaceAll('\n', ' ').replaceAll(new RegExp(r'\s+'), ' ');
 
     // Note: using "${33.0}" because the toString differs in JS vs Dart VM.
     expect(text, " Yo! The volume is ${33.0} !! The time is "
         "2013-08-08 18:34:00.000Z and here's some JSON: "
         "{here: is, some: json, x: 123} ",
         reason: 'text should match expected HTML template');
   });
-}
+});