Add requireDirective to strip-flow-types for use in React preset

[existentialism]

Q	A
Patch: Bug Fix?	N
Major: Breaking Change?	Y
Minor: New Feature?	Y
Deprecations?	N
Spec Compliancy?	N
Tests Added/Pass?	Y/Y
Fixed Tickets	Fixes #4702
License	MIT
Doc PR
Dependency Changes

Let's keep the 7.0 fun train rolling!

- Removes flow preset from preset-react, (https://twitter.com/left_pad/status/841744171723587585).

EDIT Another option would be to add a flag to flow-strip-types that will only strip types if a @flow declaration is present, and then preset-react could just pass that flag.

EDIT 2 flow-strip-types now has a requireDirective option, which preset-react uses.

• Adds development flag to preset-react as env support is being deprecated

[mention-bot]

@existentialism, thanks for your PR! By analyzing the history of the files in this pull request, we identified @danez to be a potential reviewer.

[hzoo]

cc @spicyj @samwgoldman

[danez]

Should add this to the migration guide also that flow is not included in react anymore.

[hzoo]

I do kinda like @spicyj's suggestion (mentioned in the OP now) of "Another option would be to add a flag to flow-strip-types that will only strip types if a @flow declaration is present, and then preset-react could just pass that flag."

so it's opt-in per file

[loganfsmyth]

What would people think about throwing an error if the file contains Flow annotations without @flow? This PR currently leaves them in, so people will get errors at runtime, which seems more likely to surprise people.

My motivation for wanting to remove Flow-stripping from preset-react is all about user confusion. If we make them opt in with @flow that seems totally fine to me. It does seem like it should be a build error instead though. Webpack and such will certainly error with this PR already, but if people are building for Node they won't get an error until they run the code which doesn't seem ideal.

[sophiebits]

No objections from the React side and also received positive thoughts from Flow folks on our end. This seems like a good change.

No hard opinion on @loganfsmyth's proposal but I guess erroring early sounds a little nicer. Is there a way to add the syntax-flow plugin only if the file has the directive?

[hzoo]

Is there a way to add the syntax-flow plugin only if the file has the directive?

@spicyj that particular change can't really work we can't know anything until it's parse already (so have to parse beforehand, or need to redo how that works which would reparse I think?

[loganfsmyth]

Yeah, for now instead of if (skipStrip) return; we can do if (skipStrip) throw path.buildCodeFrameError(""); at least.

[existentialism]

@hzoo @spicyj @loganfsmyth updated!

[hzoo]

I think we can explain here/somewhere that you can use this in another "env" either via "env" config in babelrc (although we didn't deprecate yet), or via babelrc.js

[hzoo]

nice work!

[hzoo]

duplicate dep here!