Remove auto-inclusion of babel-polyfill

[thymikee]

Summary

Drop auto-including of babel-polyfill if available. Fixes #2693

Test plan

jest

[wtgtybhertgeghgtwtg]

WARNING: Kneden is usable, but it's also not complete yet.

Are you sure this should go in examples?

[thymikee]

Huh, didn't notice that warning, thanks! Fallback to use babel-polyfill in that example for simplicity

[cpojer]

So.. this means regeneratorRuntime will be missing… :(

[thymikee]

Hm, so why this passes locally on all node versions?

[cpojer]

Can you wipe away all the node modules folders in examples folders and re-run yarn test?

[thymikee]

Removed couple of times from async example. Now tried again but for all examples and it fails indeed 😄 Fixed by importing babel-polyfill explicitly (this is not a great practice because it's huge, but it's easy to grasp for sure)

[cpojer]

I don't think that's great; I would expect a lot of people are using regeneratorRuntime. The alternative is to force people to compile async to generator (instead of async to generator to regenerator) but we'd need to make it so that this works in the CLI without breaking anybody or by suggesting how to do it from the CLI.

[thymikee]

compile async to generator (instead of async to generator to regenerator)

This may be tricky, because lots of folks out there are using babel-preset-es2015 which transforms to regenerator under the hood.

Not sure how to make it better.

[codecov-io]

Codecov Report

Merging #2755 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2755      +/-   ##
==========================================
+ Coverage   68.07%   68.16%   +0.09%     
==========================================
  Files         147      147              
  Lines        5347     5347              
==========================================
+ Hits         3640     3645       +5     
+ Misses       1707     1702       -5

Impacted Files	Coverage Δ
packages/jest-config/src/normalize.js	85.9% <100%> (ø)	✅
packages/jest-runtime/src/index.js	87.04% <0%> (+2.02%)	✅

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[thymikee]

So maybe we could replace babel-polyfill with regenerator-runtime only? But then again it would also affect users who rely on patching some ES6 methods with core-js instead of babel presets and plugins. I don't know.

[cpojer]

Yeah I think that would solve most use-cases. Everything in babel-polyfill is basically already in node4+.

[wtgtybhertgeghgtwtg]

Is there a reason why Travis seems to segfault a lot for this project?

[thymikee]

For sure there is, but we don't know why it happens and it's only on node 4

[cpojer]

Oh wow this is odd. Honestly, I thought I merged this into Jest 19. @thymikee mind rebasing so CI runs again?

[thymikee]

Rebased

[danny-andrews]

What does including regenerator-runtime do exactly? Because async functions don't seem to work. (I'm running in node 4.)

[thymikee]

Provides a runtime for generators. You also need a babel transform. There's a tutorial on Jest documentation

[github-actions]

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