Error after loading Firestore module

I'm having an issue with loading specific modules. It can be reproduced like this:

Install Google Firebase SDK:

```bash
npm install firebase
```

Load the modules. Please note the `firestore` add-on for Firebase, which is not captured in a variable (on purpose). It should add a `firestore()` function to the `firebase` module.

```js
import firebase from 'firebase';
import 'firebase/firestore';
```

Initialize Firestore:

```js
firebase.firestore();
```

It now throws an error:

```
Uncaught TypeError: __WEBPACK_IMPORTED_MODULE_6_firebase___default.a.firestore is not a function
```

This all worked fine with my own Webpack config, but it broke when I switched to `electron-webpack`. It seems to be doing something weird in the module loader?

Docs: https://firebase.google.com/docs/firestore/quickstart

Thanks in advance!