CSSStyleDeclaration.getPropertyValue returns empty string instead of null.

lib/html/templates/html/impl/impl_CSSStyleDeclaration.darttemplate

Index: lib/html/templates/html/impl/impl_CSSStyleDeclaration.darttemplate
===================================================================
--- lib/html/templates/html/impl/impl_CSSStyleDeclaration.darttemplate	(revision 13149)
+++ lib/html/templates/html/impl/impl_CSSStyleDeclaration.darttemplate	(working copy)
@@ -24,6 +24,14 @@
 $!MEMBERS
 
 $if DART2JS
+  String getPropertyValue(String propertyName) {
+    if (JS('bool', '!#.getPropertyValue(#)', this, propertyName)) {
+      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.