A version of the spec change from
https://github.com/tc39/ecma262/pull/988, but applied to the
Async-from-Sync iterator type.

This change does not modify generated bytecode (but maybe it should to
take advantage of load IC feedback for loading "next"). Doing this grows
bytecode by quite a bit, since it's necessary to throw-if-not-an-object
before loading "next" (which currently gets to live in a code stub
instead).

BUG=v8:5855

Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I0d2affef664d1069b24c54a553d62e17b49e5a16
Reviewed-on: https://chromium-review.googlesource.com/723136
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51078}