This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
selfBaseDuration?: number,
  
  
  
  treeBaseDuration?: number,
  
  
  
  
  _debugID?: number,
  _debugSource?: Source | null,
  _debugOwner?: Fiber | null,
  _debugIsCurrentlyTiming?: boolean,
  _debugNeedsRemount?: boolean,
  
  _debugHookTypes?: Array<HookType> | null,
|};
type BaseFiberRootProperties = {|
  
  tag: RootTag,
  
  containerInfo: any,
  
  pendingChildren: any,
  
  current: Fiber,
  pingCache: WeakMap<Wakeable, Set<mixed>> | Map<Wakeable, Set<mixed>> | null,