code: function _LightDismissableLayer_hidden(client /*: ILightDismissable */) {
                      var index = this._clients.indexOf(client);
                      if (index !== -1) {
                          this._clients.splice(index, 1);
                          client.setZIndex("");
                          client.onHide();
                          if (this._clients.length === 0) {
                              _LightDismissService.hidden(this);
                          } else {
                              _LightDismissService.updated(this);
                              this._activateTopFocusableClientIfNeeded();
                          }
                      }
                  }
docs: Dismissables should call this when they are done being dismissed (i.e. after their exit animation has finished)