Checking impl()->IsAsync(identifier) is insufficient since it could be
parenthesized. By checking the token in addition to IsIdentifier guarantees
that we've only seen the single token ASYNC.

Bug: chromium:898812
Change-Id: Id94dd607381050b4bd8cd6d8672a5d11256db7da
Reviewed-on: https://chromium-review.googlesource.com/c/1300134Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57013}