Add __DEV__ to jest environment

[KevinGrandon]

Fixes #90

[codecov]

Codecov Report

Merging #92 into master will decrease coverage by 16.99%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #92    +/-   ##
=======================================
- Coverage    67.5%   50.5%   -17%     
=======================================
  Files          69      69            
  Lines        1471    1471            
  Branches      327     327            
=======================================
- Hits          993     743   -250     
- Misses        423     690   +267     
+ Partials       55      38    -17

Impacted Files	Coverage Δ
build/jest-config.js	100% <ø> (ø)	⬆️
...s/babel-plugin-utils/replace-import-declaration.js	14.28% <0%> (-78.58%)	⬇️
...ild/babel-plugins/babel-plugin-utils/test/index.js	26.08% <0%> (-73.92%)	⬇️
...plugins/babel-plugin-experimentation/test/index.js	26.92% <0%> (-73.08%)	⬇️
...lugins/babel-plugin-i18n/test/with-translations.js	28% <0%> (-72%)	⬇️
build/babel-plugins/babel-plugin-sw/index.js	33.33% <0%> (-66.67%)	⬇️
.../babel-plugins/babel-plugin-i18n/test/translate.js	33.33% <0%> (-66.67%)	⬇️
build/babel-plugins/babel-plugin-sw/test/index.js	33.33% <0%> (-66.67%)	⬇️
...-plugins/babel-plugin-sync-chunk-ids/test/index.js	36.84% <0%> (-63.16%)	⬇️
...d/babel-plugins/babel-plugin-chunkid/test/index.js	36.84% <0%> (-63.16%)	⬇️
... and 16 more

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[KevinGrandon]

!merge

[KevinGrandon]

Zeit services are currently down it seems :( Merging manually.