Bundling failure

Hello! I'm having some issues adding this polyfill to my app. We have an ejected Expo app that is using unimodules.

Steps to Reproduce:

```
npm install @expo/browser-polyfill --save
```
In my file:
```
import '@expo/browser-polyfill';
```

error: bundling failed: Error: Unable to resolve module `expo` from `/Users/jwilliamson/Developer/QBMobile/node_modules/@expo/browser-polyfill/src/DOM/HTMLImageElement.js`: Module `expo` does not exist in the Haste module map

This might be related to https://github.com/facebook/react-native/issues/4968
To resolve try the following:
1. Clear watchman watches: `watchman watch-del-all`.
2. Delete the `node_modules` folder: `rm -rf node_modules && npm install`.
3. Reset Metro Bundler cache: `rm -rf /tmp/metro-bundler-cache-*` or `npm start -- --reset-cache`.
4. Remove haste cache: `rm -rf /tmp/haste-map-react-native-packager-*`.
```