Fixing CssStyleDeclaration to not throw exceptions for unsupported values.

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 '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 import 'dart:async';
 
@@ skipping 73 matching lines @@
     var element = new DivElement();
     document.body.children.add(element);
 
     // Need to wait one tick after the element has been added to the page.
     new Timer(const Duration(milliseconds: 10), expectAsync0(() {
       element.style.textDecoration = 'underline';
       var style = element.getComputedStyle();
       expect(style.textDecoration, equals('underline'));
     }));
   });
+
+  test('Invalid values', () {
+    var element = new DivElement();
+    // Should not throw an error.
+    element.style.background = 'some_bad_value';
+  });
 }