This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
returnFiber.flags |= ChildDeletion;
    } else {
      deletions.push(current);
    }
    newWorkInProgress.flags |= Placement;
     Restart work from the new fiber.
    return newWorkInProgress;
  } else {
    throw new Error(
      'Did not expect this call in production. ' +
        'This is a bug in React. Please file an issue.',
    );
  }
}
function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  let updateLanes = workInProgress.lanes;
  if (__DEV__) {
    if (workInProgress._debugNeedsRemount && current !== null) {
      
      return remountFiber(
        current,
        workInProgress,
        createFiberFromTypeAndProps(
          workInProgress.type,
          workInProgress.key,
          workInProgress.pendingProps,