Use head in templatizer

Author: kaste

src/lib/template/templatizer.html

@@ -325,8 +325,7 @@
 
     _notifyPathUpImpl: function(path, value) {
       var dataHost = this.dataHost;
-      var dot = path.indexOf('.');
-      var root = dot < 0 ? path : path.slice(0, dot);
+      var root = Polymer.Path.head(path);
       // Call extension point for Templatizer sub-classes
       dataHost._forwardInstancePath.call(dataHost, this, path, value);
       if (root in dataHost._parentProps) {