This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
finishedWork: Fiber | null,
  
  
  timeoutHandle: TimeoutHandle | NoTimeout,
  
  context: Object | null,
  pendingContext: Object | null,
  
  +hydrate: boolean,
  
  mutableSourceEagerHydrationData?: Array<
    MutableSource<any> | MutableSourceVersion,
  > | null,
  
  
  callbackNode: *,
  callbackPriority: Lane,
  eventTimes: LaneMap<number>,
  expirationTimes: LaneMap<number>,
  pendingLanes: Lanes,
  suspendedLanes: Lanes,
  pingedLanes: Lanes,
  expiredLanes: Lanes,
  mutableReadLanes: Lanes,
  finishedLanes: Lanes,
  entangledLanes: Lanes,
  entanglements: LaneMap<Lanes>,
  pooledCache: Cache | null,
  pooledCacheLanes: Lanes,