Fix crash in async functions when targetting ES5.

[sandersn]

Fixes #9320

When targetting ES5 and with --noImplicitReturns, an async function whose return type could not be determined would cause a compiler crash.

[sandersn]

@ahejlsberg do you want to take a look? This fix is pretty ad-hoc. But it's also only occurring when stacking a couple of wrong things on top of each other (It targets ES5 and references 'window' which doesn't exist)

[ahejlsberg]

👍

[texastoland]

But it's also only occurring when stacking a couple of wrong things on top of each other (It targets ES5 and references window which doesn't exist)

I was just trying to provide the most streamlined case possible. I'll test in the next nightly!

[sandersn]

Not a problem. I'm not sure why it didn't crash when targeting ES6 or when I substituted this for window. (Well, in the latter case, I think it was able to get a proper Promise<void> type.)

[texastoland]

In any case thanks for the patch 👏