This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
mode: TypeOfMode,
  
  flags: Flags,
  subtreeFlags: Flags,
  deletions: Array<Fiber> | null,
  
  nextEffect: Fiber | null,
  
  
  
  firstEffect: Fiber | null,
  lastEffect: Fiber | null,
  lanes: Lanes,
  childLanes: Lanes,
  
  
  
  alternate: Fiber | null,
  
  
  
  
  actualDuration?: number,
  
  
  
  actualStartTime?: number,