fix(typecheck-worker): avoid false-positive willTypechecks on Linux

[dfreeman]

We've seen occasional timeouts in CI for our test that verifies we don't block builds for file changes that don't trigger a new typecheck. Interestingly, these timeouts have never occurred on Windows, and I've also never managed to reproduce them locally on my Mac, but they pretty reliably occur every 3-4 runs on Linux.

Digging in (by running the offending test in a loop in CI), it seems that the change to a .hbs file is occasionally causing TypeScript's createProgram to be called, but then subsequently short circuited as some other piece of machinery discovers that the change in question doesn't matter. This is a problem for us, as we treat createProgram as proof that a typecheck has started, and if afterProgramCreate is never subsequently called, then our worker just sits on a pending promise forever.

Fortunately for us, when afterProgramCreate is called, it's called synchronously near the end of createProgram, so from the perspective of the outside world, it doesn't matter if we call willTypecheck in createProgram or afterProgramCreate: no events will be dispatched in between those two functions anyway.

[dfreeman]

P.S. I ran the offending test 50 times in a loop in CI and didn't get any more hangs with this change.

[chriskrycho]

I can't say I love this, but I do love that you fixed it!

[chriskrycho]

For clarification on my comment for any lookers-on/posterity: while I would guess this is fairly stable behavior, I inherently dislike solutions that leave us at the mercy of changes to an API that could change whenever convenient for the TS team—but since it's fairly core to the compiler API, it seems reasonable and we expect that it would make release notes/etc. if it were to change. And in truth, we're always in that position so…

[jamescdavis]

Thank you, @dfreeman !