Bug 1685565: Type @fluent/syntax

[Pike]

Very WIP.

Practically, I'm down to 2 flow errors, and passing tests.

Theoretically, I'm doing typing quite a bit different than it's done on the typescript side.

I remember conversations with @stasm about the use of base classes for Expressions etc and their incompleteness. Which I've done the other way around now so that refinements work.

[Pike]

This is ready for review. I left an inline comment on core/utils/fluent/types.js with an open question.

The tests expect and if not pattern is because neither flow nor typescript narrow types on failed test assertions. Thus I went for test-and-return instead.

[Pike]

Addressed the comments and discussions, TS errors are going from 137 to 105 now that we don't convert tests.

There's one in-test fix about the mocked bundles, which is one of those cases where having type checks avoids having junk in tests. Just adding that as a data point, I don't think this puts a dent in our decision to leave the tests in JavaScript.

[abowler2]

Why are these checks no longer needed? (there is the same check line#208)

[Pike]

getSyntaxType never returns an empty string, it's always one of the SyntaxType enum.

This condition will always return 'false' since the types 'SyntaxType' and '""' have no overlap.

is what https://github.com/mozilla/pontoon/runs/2520885795?check_suite_focus=true#step:16:782 says on master right now.

[mathjazz]

Flagging myself for review here to take it through my standard manual editor testing process.

[mathjazz]

I haven't spot any issues.

The code also looks good to my newbie TS self.

@abowler2 Did you want to have another look here?

[abowler2]

@Pike and @mathjazz

Took another look and all looks good to me on this one.