CSSStyleDeclaration.getPropertyValue returns empty string instead of null.

tests/html/cssstyledeclaration_test.dart

Index: tests/html/cssstyledeclaration_test.dart
===================================================================
--- tests/html/cssstyledeclaration_test.dart	(revision 13149)
+++ tests/html/cssstyledeclaration_test.dart	(working copy)
@@ -46,6 +46,15 @@
       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"));