This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
resolveClassForHotReloading,
} from './ReactFiberHotReloading.old';
import {
  mountChildFibers,
  reconcileChildFibers,
  cloneChildFibers,
} from './ReactChildFiber.old';
import {
  processUpdateQueue,
  cloneUpdateQueue,
  initializeUpdateQueue,
} from './ReactUpdateQueue.old';
import {
  NoLane,
  NoLanes,
  SyncLane,
  OffscreenLane,
  DefaultHydrationLane,
  SomeRetryLane,
  NoTimestamp,
  includesSomeLane,
  laneToLanes,
  removeLanes,
  mergeLanes,
  getBumpedLaneForHydration,
} from './ReactFiberLane.old';
import {
  ConcurrentMode,
  NoMode,
  ProfileMode,