Investigate why env setting doesn't get applied in the Babel preset

[fson]

As mentioned in #716 (comment), the env setting does not apply, unless we merge the settings from the preset manually to our config. It would be good if we could use the preset the same way with the presets option in react-scripts and ejected projects, so I'm going to investigate why this happens.

[gaearon]

It also doesn’t apply after ejecting in #716.
Weeeeird.

[gaearon]

Might be babel/babel#4539.

[gaearon]

Yeah, Babel bug. Will work around it right in the preset for now.

[gaearon]

I’ll close because not actionable for us, and I don’t care that much about using “real” env syntax because I like to enforce NODE_ENV being present.