Avoid Polymer.dom.setAttribute when unneeded.

src/standard/x-styling.html

@@ -193,7 +193,7 @@
         }
         // note: using Polymer.dom here ensures that any attribute sets
         // will provoke distribution if necessary
-        node = Polymer.dom(node);
+        node = this._clientsReadied ? Polymer.dom(node) : node;
         serializeValueToAttribute.call(this, value, attribute, node);
       },