CSSStyleDeclaration.getPropertyValue returns empty string instead of null.

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/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
@@ skipping 28 matching lines @@
     expect(style.getPropertyPriority("width"), equals("important"));
   });
 
   test('removeProperty is wrapped', () {
     var style = createTestStyle();
     style.removeProperty("width");
     expect(style.cssText.trim(),
       equals("color: blue;"));
   });
 
+  test('CSS property empty getters and setters', () {
+    var style = createTestStyle();
+    expect(style.border, equals(""));
+    
+    style.border = "1px solid blue";
+    style.border = "";
+    expect(style.border, equals(""));
+  });
+
   test('CSS property getters and setters', () {
     var style = createTestStyle();
     expect(style.color, equals("blue"));
     expect(style.width, equals("2px"));
 
     style.color = "red";
     style.transform = "translate(10px, 20px)";
 
     expect(style.color, equals("red"));
     expect(style.transform, equals("translate(10px, 20px)"));
@@ skipping 22 matching lines @@
     window.setTimeout(expectAsync0(() {
       element.style.textDecoration = 'underline';
       element.getComputedStyle('').then(expectAsync1(
         (CSSStyleDeclaration style) {
           expect(style.textDecoration, equals('underline'));
         }
       ));
     }), 10);
   });
 }