Per https://github.com/tc39/proposal-async-iteration/pull/102/files:

AsyncGeneratorResolve no longer unwraps a value component. Instead, the
value is unwrapped before the builtin call via Await, allowing Promise
rejections to affect the generator control flow.

Thus, all `yield <expr>` implicitly become `yield await <expr>`.

Additionally, `return <expr>` becomes `return await <expr>`. Finally, when
the generator is resumed with `.return()`, the parameter passed to .return()
is awaited before generator execution properly continues).

BUG=v8:5855
R=littledan@chromium.org, neis@chromium.org, adamk@chromium.org

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ife084076c3ed434b5467e6aeba14082f8b410ad5
Reviewed-on: https://chromium-review.googlesource.com/523844
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47011}