Expo support

[EvanBacon]

What kind of change does this PR introduce?

What is the current behavior?

fixes #1806

What is the new behavior?

You can now use a wide variety of Expo modules.

Checklist:

• Documentation
• Tests
• Ready to be merged
• Added myself to contributors table

[EvanBacon]

Can someone recommend a good way to test this?

[CompuIves]

Wooow, this is awesome!! Thanks a lot @EvanBacon! We deployed the PR to here: http://pr1807.cs.lbogdan.tk/s/new. There you can also test if you can add expo modules. I'd be glad to help get this through!

[EvanBacon]

@CompuIves thanks for the deploy! The aliases seem to work as expected. It looks like there is a bug where .web extensions aren't being used in node modules.

// This works: `expo-asset/build/AssetRegistry.web.js`
import * as AssetRegistry from 'expo-asset/build/AssetRegistry.web'

// This doesn't, it resolves to: `expo-asset/build/AssetRegistry.js`
// which also exists but is being used for the native functionality
// essentially returning an empty object as it's looking for a React Native `NativeModule`. 
import * as AssetRegistry from 'expo-asset/build/AssetRegistry'

[CompuIves]

Ahh, I see! I pushed a fix to master, if you merge it should work (on staging at least).

[EvanBacon]

This works really well! Still some bugs that I can work out in the individual modules. One more thing that would be nice is the environment variable __DEV__ that Metro provides. Not sure if this will conflict with anything on your end though, we do this in the expo/webpack-config here to better support libraries that aren't fully optimized for web.

[EvanBacon]

Given that __DEV__ is not an actual blocker, I think it would make sense to merge this PR as is unless @CompuIves has any other concerns.

[CompuIves]

That's great!! I'll merge this in tomorrow morning!

Really cool to have Expo support soon in CodeSandbox, I've been meaning for a long time to look into it. Thank you so much for contributing this @EvanBacon!