Mark some ephemeral super classes as private.

lib/elements/dom-bind.html

@@ -24,6 +24,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @implements {Polymer_PropertyEffects}
      * @implements {Polymer_OptionalMutableData}
      * @implements {Polymer_GestureEventListeners}
+     * @private
      */
     const domBindBase =
       Polymer.GestureEventListeners(

lib/elements/dom-repeat.html

@@ -24,6 +24,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
    * @constructor
    * @implements {Polymer_OptionalMutableData}
    * @extends {Polymer.Element}
+   * @private
    */
   const domRepeatBase = Polymer.OptionalMutableData(Polymer.Element);