Dont path a path object into PolymerExpressions.getBinding

https://codereview.appspot.com/95740044/ broken PolymerExpressions by handing in a Path object rather than the pathString.

R=sorvell
BUG=

Review URL: https://codereview.appspot.com/91770043

Author: rafaelw

src/declaration/mdv.js

@@ -17,7 +17,7 @@
     var path = Path.get(pathString);
 
     return events.prepareEventBinding(path, name, node) ||
-           prepareBinding.call(syntax, path, name, node);
+           prepareBinding.call(syntax, pathString, name, node);
   };
 
   // declaration api supporting mdv