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.
export const AsyncMode = REACT_CONCURRENT_MODE_TYPE;
export const ConcurrentMode = REACT_CONCURRENT_MODE_TYPE;
export const ContextConsumer = REACT_CONTEXT_TYPE;
export const ContextProvider = REACT_PROVIDER_TYPE;
export const Element = REACT_ELEMENT_TYPE;
export const ForwardRef = REACT_FORWARD_REF_TYPE;
export const Fragment = REACT_FRAGMENT_TYPE;
export const Profiler = REACT_PROFILER_TYPE;
export const Portal = REACT_PORTAL_TYPE;
export const StrictMode = REACT_STRICT_MODE_TYPE;
export {isValidElementType};
let hasWarnedAboutDeprecatedIsAsyncMode = false;
export function isAsyncMode(object: any) {
  if (__DEV__) {
    if (!hasWarnedAboutDeprecatedIsAsyncMode) {
      hasWarnedAboutDeprecatedIsAsyncMode = true;
      lowPriorityWarning(
        false,
        'The ReactIs.isAsyncMode() alias has been deprecated, ' +
          'and will be removed in React 17+. Update your code to use ' +
          'ReactIs.isConcurrentMode() instead. It has the exact same API.',
      );
    }
  }
  return isConcurrentMode(object);
}
export function isConcurrentMode(object: any) {
  return typeOf(object) === REACT_CONCURRENT_MODE_TYPE;
}
export function isContextConsumer(object: any) {