code: function _addEventDispatcherImpl(proto) {
          var temp = {};
          EventDispatcher.makeEventDispatcher(temp);
          proto._on_internal  = temp.on;
          proto._off_internal = temp.off;
          proto.trigger       = temp.trigger;
      }
docs: Utility for PreferencesSystem & PrefixedPreferencesSystem -- attach EventDispatcher's on()/off()
  implementation as private _on_internal()/_off_internal() methods, so the custom on()/off() APIs
  these classes use can leverage EventDispatcher code internally. Also attach the regular public trigger().