Currently, async generators are stringified the same way normal
Generators are. This change prefixes async generator methods with
"async *", and other async generator functions with
"async function* ".

BUG=v8:5855
R=adamk@chromium.org, littledan@chromium.org, jwolfe@igalia.com

Change-Id: Ia809fad64caac4464dbc9f7fa7728584d0f67832
Reviewed-on: https://chromium-review.googlesource.com/463526
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44278}