This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
return: Fiber | null,
  
  child: Fiber | null,
  sibling: Fiber | null,
  index: number,
  
  
  ref:
    | null
    | (((handle: mixed) => void) & {_stringRef: ?string, ...})
    | RefObject,
  
  pendingProps: any, 
  memoizedProps: any, 
  
  updateQueue: mixed,
  
  memoizedState: any,
  
  dependencies: Dependencies | null,