buttonDown method Null safety

Implementation

@override
void buttonDown(Operation key) {
  if (key == controller.gotoLineNumberKey) {
    changeState(WaitingForGotoDotLines(controller, last));
  } else {
    changeState(ArgInputState(controller.gtoOperation,
            controller.gtoOperation.arg, controller, last))
        .buttonDown(key);
    // Not controller.runWithArg().  We need to invoke buttonDown, which can't
    // be done with a RunningController.  In the RunningController case, we
    // never go into the WaitingForGotoDot state -- see assert in
    // constructor.
  }
}