This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
if (__DEV__) {
    owner = element._owner;
  }
  const type = element.type;
  const key = element.key;
  const pendingProps = element.props;
  const fiber = createFiberFromTypeAndProps(
    type,
    key,
    pendingProps,
    owner,
    mode,
    lanes,
  );
  if (__DEV__) {
    fiber._debugSource = element._source;
    fiber._debugOwner = element._owner;
  }
  return fiber;
}
export function createFiberFromFragment(
  elements: ReactFragment,
  mode: TypeOfMode,
  lanes: Lanes,
  key: null | string,
): Fiber {
  const fiber = createFiber(Fragment, elements, key, mode);
  fiber.lanes = lanes;
  return fiber;
}