It is a Jest-ism that shouldn't be in our open source package.
People using Jest can implement a similar pattern directly.
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
import React from 'react';
import ReactDOM from 'react-dom';
import {findCurrentFiberUsingSlowPath} from 'react-reconciler/reflection';
import * as ReactInstanceMap from 'shared/ReactInstanceMap';
import {
  ClassComponent,
  FunctionalComponent,
  HostComponent,
  HostText,
} from 'shared/ReactTypeOfWork';
import SyntheticEvent from 'events/SyntheticEvent';
import invariant from 'shared/invariant';
import * as DOMTopLevelEventTypes from '../events/DOMTopLevelEventTypes';
const {findDOMNode} = ReactDOM;
const {
  EventPluginHub,
  EventPluginRegistry,
  EventPropagators,
  ReactControlledComponent,
  ReactDOMComponentTree,
  ReactDOMEventListener,
} = ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
function Event(suffix) {}