Syncing CssStyleDeclaration.setPropertyValue's signature between dart2js and dartium

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 72411293646d15e7faecc87ec3eba50eec181bb1..9fa473957ffad81f8ff319a2ecc2fe08507c65ff 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -2604,7 +2604,7 @@ class CssStyleDeclaration extends NativeFieldWrapperClass1 {
 
   @DomName('CSSStyleDeclaration.setProperty')
   @DocsEditable
-  void setProperty(String propertyName, String value, String priority) native "CSSStyleDeclaration_setProperty_Callback";
+  void _setProperty(String propertyName, String value, String priority) native "CSSStyleDeclaration_setProperty_Callback";
 
 
   String getPropertyValue(String propertyName) {
@@ -2617,6 +2617,14 @@ class CssStyleDeclaration extends NativeFieldWrapperClass1 {
    */
   static bool get supportsTransitions => true;
 
+  @DomName('CSSStyleDeclaration.setProperty')
+  void setProperty(String propertyName, String value, [String priority]) {
+    if (priority == null) {
+      priority = '';
+    }
+    _setProperty(propertyName, value, priority);
+  }
+
   // TODO(jacobr): generate this list of properties using the existing script.
   /** Gets the value of "align-content" */
   String get alignContent =>