Syncing CssStyleDeclaration.setPropertyValue's signature between dart2js and dartium

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev' hide Symbol;
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
@@ skipping 2154 matching lines @@
   @DomName('CSSStyleDeclaration.removeProperty')
   @DocsEditable
   String removeProperty(String propertyName) native;
 
 
   String getPropertyValue(String propertyName) {
     var propValue = _getPropertyValue(propertyName);
     return propValue != null ? propValue : '';
   }
 
+  @DomName('CSSStyleDeclaration.setProperty')
   void setProperty(String propertyName, String value, [String priority]) {
     // try/catch for IE9 which throws on unsupported values.
     try {
+      if (priority == null) {
+        priority = '';
+      }
       JS('void', '#.setProperty(#, #, #)', this, propertyName, value, priority);
       // Bug #2772, IE9 requires a poke to actually apply the value.
       if (JS('bool', '!!#.setAttribute', this)) {
         JS('void', '#.setAttribute(#, #)', this, propertyName, value);
       }
     } catch (e) {}
   }
 
   /**
    * Checks to see if CSS Transitions are supported.
@@ skipping 25746 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }