docs: function modalDialog_start(aCallback) {
      if (!aCallback)
        throw new Error(arguments.callee.name + ": Callback not specified.");
  
      this._observer = new mdObserver(this._window, aCallback);
  
      this._timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
      this._timer.init(this._observer, DELAY_CHECK, Ci.nsITimer.TYPE_ONE_SHOT);
    }