instance.$.foo should only give Elements

You need to be at least an `Element` to have an id attribute, and you need to have an id attribute to be in the `$` map.

Author: rictic

externs/closure-types.js

@@ -693,7 +693,7 @@ Polymer_ElementMixin.prototype.importPath;
 /** @type {(StampedTemplate|HTMLElement|ShadowRoot)} */
 Polymer_ElementMixin.prototype.root;
 
-/** @type {!Object.<string, !Node>} */
+/** @type {!Object.<string, !Element>} */
 Polymer_ElementMixin.prototype.$;
 
 /**
@@ -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){};