Annotate Node parameter as not null.

lib/mixins/template-stamp.js

@@ -466,7 +466,7 @@ export const TemplateStamp = dedupingMixin(superClass => {
     /**
      * 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}