This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
import type {ReactProviderType, ReactContext} from 'shared/ReactTypes';
import type {LazyComponent as LazyComponentType} from 'react/src/ReactLazy';
import type {Fiber} from './ReactInternalTypes';
import type {FiberRoot} from './ReactInternalTypes';
import type {Lanes, Lane} from './ReactFiberLane.old';
import type {MutableSource} from 'shared/ReactTypes';
import type {
  SuspenseState,
  SuspenseListRenderState,
  SuspenseListTailMode,
} from './ReactFiberSuspenseComponent.old';
import type {SuspenseContext} from './ReactFiberSuspenseContext.old';
import type {
  OffscreenProps,
  OffscreenState,
} from './ReactFiberOffscreenComponent';
import type {
  Cache,
  CacheComponentState,
  SpawnedCachePool,
} from './ReactFiberCacheComponent.old';
import type {UpdateQueue} from './ReactUpdateQueue.old';
import checkPropTypes from 'shared/checkPropTypes';
import {
  IndeterminateComponent,
  FunctionComponent,
  ClassComponent,
  HostRoot,
  HostComponent,
  HostText,