Implement type change in Polymer.ElementMixin

Author: dfreedm

externs/closure-types.js

@@ -1210,4 +1210,4 @@ Polymer_ArraySelectorMixin.prototype.select = function(item){};
 /**
 * @param {number} idx Index from `items` array to select
 */
-Polymer_ArraySelectorMixin.prototype.selectIndex = function(idx){};
+Polymer_ArraySelectorMixin.prototype.selectIndex = function(idx){};

lib/mixins/element-mixin.html

@@ -526,7 +526,7 @@
         this.importPath;
         /** @type {StampedTemplate | HTMLElement | ShadowRoot} */
         this.root;
-        /** @type {!Object<string, !Node>} */
+        /** @type {!Object<string, !Element>} */
         this.$;
       }