This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
export function createFiberFromOffscreen(
  pendingProps: OffscreenProps,
  mode: TypeOfMode,
  lanes: Lanes,
  key: null | string,
) {
  const fiber = createFiber(OffscreenComponent, pendingProps, key, mode);
  fiber.elementType = REACT_OFFSCREEN_TYPE;
  fiber.lanes = lanes;
  return fiber;
}
export function createFiberFromLegacyHidden(
  pendingProps: OffscreenProps,
  mode: TypeOfMode,
  lanes: Lanes,
  key: null | string,
) {
  const fiber = createFiber(LegacyHiddenComponent, pendingProps, key, mode);
  fiber.elementType = REACT_LEGACY_HIDDEN_TYPE;
  fiber.lanes = lanes;
  return fiber;
}
export function createFiberFromCache(
  pendingProps: any,
  mode: TypeOfMode,
  lanes: Lanes,
  key: null | string,
) {
  const fiber = createFiber(CacheComponent, pendingProps, key, mode);
  fiber.elementType = REACT_CACHE_TYPE;