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.
if (typeof object === 'object' && object !== null) {
    const $$typeof = object.$$typeof;
    switch ($$typeof) {
      case REACT_ELEMENT_TYPE:
        const type = object.type;
        switch (type) {
          case REACT_CONCURRENT_MODE_TYPE:
          case REACT_FRAGMENT_TYPE:
          case REACT_PROFILER_TYPE:
          case REACT_STRICT_MODE_TYPE:
            return type;
          default:
            const $$typeofType = type && type.$$typeof;
            switch ($$typeofType) {
              case REACT_CONTEXT_TYPE:
              case REACT_FORWARD_REF_TYPE:
              case REACT_PROVIDER_TYPE:
                return $$typeofType;
              default:
                return $$typeof;
            }
        }
      case REACT_PORTAL_TYPE:
        return $$typeof;
    }
  }
  return undefined;
}