Got this debugging another error. Filing here for reference. Original code: https://phabricator.intern.facebook.com/P59317705
/**
 * 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 React = require('react');
let ReactDOM = require('react-dom');
let ReactDOMServer = require('react-dom/server');
let AsyncMode = React.unstable_AsyncMode;
describe('ReactDOMRoot', () => {
  let container;
  let scheduledCallback;
  let flush;
  let now;
  let expire;
  beforeEach(() => {
    container = document.createElement('div');
    
    scheduledCallback = null;
    flush = function(units = Infinity) {
      if (scheduledCallback !== null) {
        let didStop = false;