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.
 *
 * @emails react-core
 */
'use strict';
let createRenderer;
let React;
let ReactDOM;
let ReactDOMServer;
let ReactTestUtils;
function getTestDocument(markup) {