Allow non-exported process.env vars to be changed in browser.
Allow non-exported process.env vars to be changed in browser.

This gets rid of the warnings coming out of the browser tests because
`process.env.NODE_ENV = 'test'` in the browser tests was effectively
a no-op (due to rewriting caused by webpack's DefinePlugin).

mmmavis/teach.webmaker.org,Pomax/teach.webmaker.org,Pomax/teach.webmaker.org,emmairwin/teach.webmaker.org,Pomax/teach.mozilla.org,mozilla/teach.mozilla.org,mozilla/teach.webmaker.org,asdofindia/teach.webmaker.org,mmmavis/teach.webmaker.org,mrinaljain/teach.webmaker.org,fredericksilva/teach.webmaker.org,mmmavis/teach.mozilla.org,AnthonyBobsin/teach.webmaker.org,cadecairos/teach.mozilla.org,emmairwin/teach.webmaker.org,toolness/teach.webmaker.org,mozilla/learning.mozilla.org,fredericksilva/teach.webmaker.org,mozilla/teach.mozilla.org,mrinaljain/teach.webmaker.org,asdofindia/teach.webmaker.org,alicoding/teach.mozilla.org,mmmavis/teach.mozilla.org,AnthonyBobsin/teach.webmaker.org,ScottDowne/teach.webmaker.org,mozilla/teach.webmaker.org,alicoding/teach.mozilla.org,PaulaPaul/teach.mozilla.org,Pomax/teach.mozilla.org,alicoding/teach.webmaker.org,toolness/teach.webmaker.org,ScottDowne/teach.webmaker.org,PaulaPaul/teach.mozilla.org,alicoding/teach.webmaker.org,cadecairos/teach.mozilla.org