This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
useContext<T>(context: ReactContext<T>): T,
  useRef<T>(initialValue: T): {|current: T|},
  useEffect(
    create: () => (() => void) | void,
    deps: Array<mixed> | void | null,
  ): void,
  useLayoutEffect(
    create: () => (() => void) | void,
    deps: Array<mixed> | void | null,
  ): void,
  useCallback<T>(callback: T, deps: Array<mixed> | void | null): T,
  useMemo<T>(nextCreate: () => T, deps: Array<mixed> | void | null): T,
  useImperativeHandle<T>(
    ref: {|current: T | null|} | ((inst: T | null) => mixed) | null | void,
    create: () => T,
    deps: Array<mixed> | void | null,
  ): void,
  useDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void,
  useDeferredValue<T>(value: T): T,
  useTransition(): [boolean, (() => void) => void],
  useMutableSource<Source, Snapshot>(
    source: MutableSource<Source>,
    getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
    subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
  ): Snapshot,
  useOpaqueIdentifier(): any,
  useCacheRefresh?: () => <T>(?() => T, ?T) => void,
  unstable_isNewReconciler?: boolean,
|};