Add property to CssStyleDeclaration to detect if a CSS value is available.

tests/html/cssstyledeclaration_test.dart

 // Copyright (c) 2011, 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 CssStyleDeclarationTest;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import 'dart:async';
 import 'utils.dart';
@@ skipping 132 matching lines @@
     elements = document.queryAll('.bar');
     expect(elements.style.backgroundColor, equals('green'));
     elements = document.queryAll('#wat');
     expect(elements.style.backgroundColor, equals('yellow'));
 
     elements.style.borderLeftWidth = '18px';
     expect(elements.style.borderLeftWidth, equals('18px'));
     elements = document.queryAll('li');
     expect(elements.style.borderLeftWidth, equals('10px'));
   });
+
+  test('supports property', () {
+    expect(document.body.style.supportsProperty('bogus-property'), false);
+    expect(document.body.style.supportsProperty('background'), true);
+    expect(document.body.style.supportsProperty('borderBottomWidth'), true);
+    expect(document.body.style.supportsProperty('animation'), true);
+  });
 }