This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
|};
type UpdaterTrackingOnlyFiberRootProperties = {|
  memoizedUpdaters: Set<Fiber>,
  pendingUpdatersLaneMap: LaneMap<Set<Fiber>>,
|};
export type SuspenseHydrationCallbacks = {
  onHydrated?: (suspenseInstance: SuspenseInstance) => void,
  onDeleted?: (suspenseInstance: SuspenseInstance) => void,
  ...
};
type SuspenseCallbackOnlyFiberRootProperties = {|
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
|};
export type FiberRoot = {
  ...BaseFiberRootProperties,
  ...SuspenseCallbackOnlyFiberRootProperties,
  ...UpdaterTrackingOnlyFiberRootProperties,
  ...
};
type BasicStateAction<S> = (S => S) | S;
type Dispatch<A> = A => void;
export type Dispatcher = {|
  getCacheForType?: <T>(resourceType: () => T) => T,
  readContext<T>(context: ReactContext<T>): T,
  useState<S>(initialState: (() => S) | S): [S, Dispatch<BasicStateAction<S>>],
  useReducer<S, I, A>(
    reducer: (S, A) => S,
    initialArg: I,
    init?: (I) => S,
  ): [S, Dispatch<A>],