void lockState(void callback())

Establishes a scope in which calls to State.setState are forbidden, and calls the given callback.

This mechanism is used to ensure that, for instance, State.dispose does not call State.setState.

Source 

void lockState(void callback()) {
  assert(callback != null);
  assert(_debugStateLockLevel >= 0);
  assert(() {
    _debugStateLockLevel += 1;
    return true;
  });
  try {
    callback();
  } finally {
    assert(() {
      _debugStateLockLevel -= 1;
      return true;
    });
  }
  assert(_debugStateLockLevel >= 0);
}