Browser platform: "The XMLHttpRequest compatibility library was not found."

I was having this error:

```
auth.esm.js:139 Uncaught Ui {code: "auth/internal-error", message: "The XMLHttpRequest compatibility library was not found."}
```

And the workaround for it to dissappear is:

```javascript
window.global = window; // hack for 'FirebaseUIAuth'
```

Hope this helps if anyone is having the same issue. This may be useful to add in the README file.