update elements and fix some bugs

test/paper_button_test.dart

 // Copyright (c) 2015, 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.
 @TestOn('browser')
 library polymer_elements.test.paper_button_test;
 
 import 'dart:async';
 import 'package:polymer_elements/paper_button.dart';
 import 'package:web_components/web_components.dart';
 import 'package:test/test.dart';
 import 'common.dart';
 
 main() async {
   await initWebComponents();
 
   group('<paper-button>', () {
     PaperButton button;
     setUp(() {
       button = fixture('TrivialButton');
     });
 
     test('can be raised imperatively', () {
       Completer done = new Completer();
       button.raised = true;
       expect(button.getAttribute('raised'), isNotNull);
       wait(1).then((_) {
         try {
-          expect(button.jsElement['_elevation'], equals(1));
+          expect(button.elevation, equals(1));
           done.complete();
         } catch (e) {
           done.complete(e);
         }
       });
     });
 
     test('can be disabled imperatively', () {
       button.disabled = true;
       expect(button.getAttribute('aria-disabled'), equals('true'));
@@ skipping 29 matching lines @@
     });
 
     test('has aria role "button"', () {
       expect(button.getAttribute('role'), 'button');
     });
 
     // TODO(jakemac): What is this?
     // a11ySuite('TrivialButton');
   });
 }