It seems like react-is v16.6 no longer is compatible with React v16.3 - v16.5, in that React v16.3 - v16.5 will use `Symbol(react.async_mode)`, but `react-is`'s `AsyncMode` export will instead be `Symbol(react.concurrent_mode)`.

It seems like the async mode symbol needs to remain the same in react-is 16.6+, and the `isAsyncMode` function would need to check *both* the AsyncMode and ConcurrentMode symbols.
/**
 * 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.
 *
 * @emails react-core
 */
'use strict';
let React;
let ReactDOM;
let ReactIs;
describe('ReactIs', () => {
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
    ReactIs = require('react-is');
  });
  it('should return undefined for unknown/invalid types', () => {
    expect(ReactIs.typeOf('abc')).toBe(undefined);
    expect(ReactIs.typeOf(true)).toBe(undefined);
    expect(ReactIs.typeOf(123)).toBe(undefined);
    expect(ReactIs.typeOf({})).toBe(undefined);
    expect(ReactIs.typeOf(null)).toBe(undefined);
    expect(ReactIs.typeOf(undefined)).toBe(undefined);
  });
  it('identifies valid element types', () => {
    class Component extends React.Component {