Allow Polymer fn's call to Class to be overridden.

lib/legacy/polymer-fn.js

@@ -28,15 +28,19 @@ import '../utils/boot.js';
  * @return {function(new: HTMLElement)} Generated class
  * @suppress {duplicate, invalidCasts, checkTypes}
  */
-export const Polymer = function(info) {
+const Polymer = function(info) {
   // if input is a `class` (aka a function with a prototype), use the prototype
   // remember that the `constructor` will never be called
   let klass;
   if (typeof info === 'function') {
     klass = info;
   } else {
-    klass = Class(info);
+    klass = Polymer.Class(info);
   }
   customElements.define(klass.is, /** @type {!HTMLElement} */(klass));
   return klass;
 };
+
+Polymer.Class = Class;
+
+export { Polymer };