Corrects a small problem with the current implementation of the AsyncFunction
constructor.

See https://tc39.github.io/ecmascript-asyncawait/#async-function-constructor for
details.

BUG=v8:4483
R=littledan@chromium.org, adamk@chromium.org, jwolfe@igalia.com

Review-Url: https://codereview.chromium.org/2118653004
Cr-Commit-Position: refs/heads/master@{#37484}