`async` keyword missing from completion list when nested inside another function

[AlCalzone]

TypeScript Version: 3.1.0-dev.20180717

Search Terms: async completion suggestion

When inside another function, the async keyword no longer appears in completion lists. This often happens when writing async tests using mocha. See the following screenshots for a demonstration.

Not nested, async is suggested:

Nested, async is not suggested:

Expected behavior:
async is suggested in both cases

Actual behavior:
It is not

[mhegazy]

PRs welcomed.