Fixing CssStyleDeclaration to not throw exceptions for unsupported values.

tests/html/cssstyledeclaration_test.dart

Index: tests/html/cssstyledeclaration_test.dart
diff --git a/tests/html/cssstyledeclaration_test.dart b/tests/html/cssstyledeclaration_test.dart
index 1f8660532d9b262095424f1876efe0fbdef8c464..13a9730a7c02703b44f4dc187be88d03f8935ac6 100644
--- a/tests/html/cssstyledeclaration_test.dart
+++ b/tests/html/cssstyledeclaration_test.dart
@@ -91,4 +91,10 @@ main() {
       expect(style.textDecoration, equals('underline'));
     }));
   });
+
+  test('Invalid values', () {
+    var element = new DivElement();
+    // Should not throw an error.
+    element.style.background = 'some_bad_value';
+  });
 }