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.
expect(ReactIs.isElement(<div />)).toBe(true);
    expect(ReactIs.isElement('div')).toBe(false);
    expect(ReactIs.isElement(true)).toBe(false);
    expect(ReactIs.isElement(123)).toBe(false);
    expect(ReactIs.isElement(null)).toBe(false);
    expect(ReactIs.isElement(undefined)).toBe(false);
    expect(ReactIs.isElement({})).toBe(false);
    
    const Context = React.createContext(false);
    expect(ReactIs.isElement(<Context.Provider />)).toBe(true);
    expect(ReactIs.isElement(<Context.Consumer />)).toBe(true);
    expect(ReactIs.isElement(<React.Fragment />)).toBe(true);
    expect(ReactIs.isElement(<React.unstable_ConcurrentMode />)).toBe(true);
    expect(ReactIs.isElement(<React.StrictMode />)).toBe(true);
  });
  it('should identify ref forwarding component', () => {
    const RefForwardingComponent = React.forwardRef((props, ref) => null);
    expect(ReactIs.typeOf(<RefForwardingComponent />)).toBe(ReactIs.ForwardRef);
    expect(ReactIs.isForwardRef(<RefForwardingComponent />)).toBe(true);
    expect(ReactIs.isForwardRef({type: ReactIs.StrictMode})).toBe(false);
    expect(ReactIs.isForwardRef(<React.unstable_ConcurrentMode />)).toBe(false);
    expect(ReactIs.isForwardRef(<div />)).toBe(false);
  });
  it('should identify fragments', () => {
    expect(ReactIs.typeOf(<React.Fragment />)).toBe(ReactIs.Fragment);
    expect(ReactIs.isFragment(<React.Fragment />)).toBe(true);
    expect(ReactIs.isFragment({type: ReactIs.Fragment})).toBe(false);
    expect(ReactIs.isFragment('React.Fragment')).toBe(false);
    expect(ReactIs.isFragment(<div />)).toBe(false);
    expect(ReactIs.isFragment([])).toBe(false);