Annotate Node parameter as not null.

lib/mixins/template-stamp.html

@@ -472,7 +472,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       /**
        * Override point for adding custom or simulated event handling.
        *
-       * @param {Node} node Node to remove event listener from
+       * @param {!Node} node Node to remove event listener from
        * @param {string} eventName Name of event
        * @param {function(!Event):void} handler Listener function to remove
        * @return {void}