Upgrade to RN v0.64

[gnprice]

The next round after #4245. This is the first time I'm opening one of these when we're already on the previous latest release, and before the new one is even out yet!

The RN community release managers are working on the next release now, and it's anticipated in the next few weeks. Details at react-native-community/releases#214 and particularly at react-native-community/releases#214 (comment) .

There's already a release candidate, so that would be a way to get an early check on any issues we'll run into.

[chrisbobbe]

We'll be jumping to Flow 0.137.0, which means (among other things) that something called "types-first" has been both introduced and enabled by default. We can turn it off, though, by adding types_first=false under [options] in our Flow config.

The thing we'll have to do to enable it is annotate almost all the exports of our modules:

The caveat of this new version is that it requires exported parts of the code to be annotated with types, or to be expressions whose type can be trivially inferred (for example numbers and strings).

Not annotating an export sufficiently will raise a signature-verification-failure error.

It looks like we'll be able to work toward that incrementally by selecting certain directories where we want to check the exports of the contained modules.

They mention a codemod, but I suspect, if we use it at all, we'll want to use it on small sections of the code, one at a time, and check its work.

[gnprice]

We'll be jumping to Flow 0.137.0, which means (among other things) that something called "types-first" has been both introduced and enabled by default

Cool! The performance benefits of that won't be as big for us as they are for a giant codebase like Facebook's, but we should see some. I think the quality of the type errors may improve, too.

Re the codemod, we'll definitely want to check its work: making readable commits and reviewing them normally. I suspect we'll want to start by trying it on relatively self-contained swathes of code like src/api/ or even src/third/; maybe then reducers+selectors, the other files that don't know about React, and finally swathes of React components. Depending on how much manual work is needed (and how much effort is needed to review the output), we can scale up or down how much code we cover per commit.

And if the codemod is really bad at its job, we can always ignore it and do the changes manually. But I'm hopeful that it'll be good enough, automating enough boring stuff accurately enough, that it'll be worth using.

---

FTR here's the full list of changes we'll see -- up to this point in the changelog:
https://github.com/facebook/flow/blob/master/Changelog.md#01370
from 0.122.0, which we're currently on.

Other changes I see in a quick scan which we might appreciate, or might have to adapt for:

• Allow indexers in exact object annotations

This should let us clean up a few types.

• Standardized error suppression syntax, added ability to suppress errors based on error codes

The second part of this is #4433. The first means we'll have to drop $FlowMigrationFudge and replace it with one of the standard names.

• Enable LSP support for autofix exports by default.
• Enabled support for JSDoc in some LSP results.
• Show JSDoc documentation in hover, autocomplete, and signature help.

I'm hopeful these will translate to some nice improvements in the VS Code editing experience!

• Fixed Object.freeze to no longer incorrectly convert an inexact object into an exact object.

Hmm, I wonder if this will defeat the workaround we've used for a Flow bug involving {} as an "unsealed" object type.

[gnprice]

We can turn it off, though, by adding types_first=false under [options] in our Flow config.

Oh also: this might be exactly the right thing to do for the RN upgrade. We'll want to do this if the types-first migration seems to be any substantial amount of work, I think.

But then we'll want to follow up by doing the types-first migration. A newer version of Flow (v0.143.0, out just this month) makes that the only available mode, so we'll presumably get that change in the next version of RN, v0.65.