CSSStyleDeclaration.getPropertyValue returns empty string instead of null.

lib/html/dart2js/html_dart2js.dart

Index: lib/html/dart2js/html_dart2js.dart
===================================================================
--- lib/html/dart2js/html_dart2js.dart	(revision 13149)
+++ lib/html/dart2js/html_dart2js.dart	(working copy)
@@ -4334,8 +4334,6 @@
 
   String getPropertyShorthand(String propertyName) native;
 
-  String getPropertyValue(String propertyName) native;
-
   bool isPropertyImplicit(String propertyName) native;
 
   String item(int index) native;
@@ -4343,6 +4341,14 @@
   String removeProperty(String propertyName) native;
 
 
+  String getPropertyValue(String propertyName) {
+    if (JS('bool', '!#.getPropertyValue(#)', this, propertyName)) {

[blois, 2012/10/03 00:56:47]

This is getting the property value twice- should probably get it once, check if
it's null and return empty string if so.

+      return '';
+    } else {
+      return JS('String', '#.getPropertyValue(#)', this, propertyName);
+    }
+  }
+
   void setProperty(String propertyName, String value, [String priority]) native '''
     this.setProperty(propertyName, value, priority);
     // Bug #2772, IE9 requires a poke to actually apply the value.