Adds process.env.NODE_ENV polyfill

[dsibiski]

There are many libraries that use NODE_ENV to check whether the code is running in
"production" mode or not. This allows those library authors to not have to add conditionals
for React Native.

One such library is Redux: reduxjs/react-redux#40

Thanks to @brentvatne for providing the solution via this tweet (via his phone in the airport 😉): https://twitter.com/notbrent/status/630440250951749632

/cc @vjeux @gaearon @zpao @amasad

[vjeux]

@facebook-github-bot import

[facebook-github-bot]

Thanks for importing. If you are an FB employee go to https://our.intern.facebook.com/intern/opensource/github/pull_request/899585296789195/int_phab to review.

[vjeux]

While looking into it, turns out that we don't currently remove if (__DEV__) { } blocks from the minified version. I'm working on fixing it as well.

[gaearon]

@dsibiski Thank you!

[vjeux]

Pull request opened on UglifyJS: mishoo/UglifyJS#767

[amasad]

we also now inline __DEV__ and NODE_ENV checks so that the minifier can safely remove them:

react-native/packager/transformer.js

Line 19 in b9049ee

plugins = plugins.concat(['node-env-inline', 'dunderscore-dev-inline']);