AnonymousAuthProvider signInOptions does not work; Throw a TypeError.

- Using `AnonymousAuthProvider` signInOptions does not work with `react-firebaseui`.
- It throws the following error: `Uncaught TypeError: fn(...).signInAnonymously is not a function`

Below is what my `uiConfig` looks like.

```
import { auth as firebaseuiAuth } from 'firebaseui';

const uiConfig = {
signInFlow: 'popup',
signInOptions: [
firebaseuiAuth.AnonymousAuthProvider.PROVIDER_ID,
],
};
```