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.
render() {
        return React.createElement('div');
      }
    }
    const FunctionComponent = () => React.createElement('div');
    const ForwardRefComponent = React.forwardRef((props, ref) =>
      React.createElement(Component, {forwardedRef: ref, ...props}),
    );
    const Context = React.createContext(false);
    expect(ReactIs.isValidElementType('div')).toEqual(true);
    expect(ReactIs.isValidElementType(Component)).toEqual(true);
    expect(ReactIs.isValidElementType(FunctionComponent)).toEqual(true);
    expect(ReactIs.isValidElementType(ForwardRefComponent)).toEqual(true);
    expect(ReactIs.isValidElementType(Context.Provider)).toEqual(true);
    expect(ReactIs.isValidElementType(Context.Consumer)).toEqual(true);
    expect(ReactIs.isValidElementType(React.createFactory('div'))).toEqual(
      true,
    );
    expect(ReactIs.isValidElementType(React.Fragment)).toEqual(true);
    expect(ReactIs.isValidElementType(React.unstable_ConcurrentMode)).toEqual(
      true,
    );
    expect(ReactIs.isValidElementType(React.StrictMode)).toEqual(true);
    expect(ReactIs.isValidElementType(true)).toEqual(false);
    expect(ReactIs.isValidElementType(123)).toEqual(false);
    expect(ReactIs.isValidElementType({})).toEqual(false);
    expect(ReactIs.isValidElementType(null)).toEqual(false);
    expect(ReactIs.isValidElementType(undefined)).toEqual(false);
    expect(ReactIs.isValidElementType({type: 'div', props: {}})).toEqual(false);