function _addEventDispatcherImpl(proto) {
        var temp = {};
        EventDispatcher.makeEventDispatcher(temp);
        proto._on_internal  = temp.on;
        proto._off_internal = temp.off;
        proto.trigger       = temp.trigger;
    }
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().