This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import type {Source} from 'shared/ReactElementType';
import type {
  RefObject,
  ReactContext,
  MutableSourceSubscribeFn,
  MutableSourceGetSnapshotFn,