In the contexts for the resolver closures used in Promise.all we can
save the "already visited" cell, by just setting the index slot to a
negative value, which then indicates that this element was already
done.

Bug: v8:7253
Change-Id: I1296a2216eac3b51368c1e7795dbcd2c80cc430a
Reviewed-on: https://chromium-review.googlesource.com/903928Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51125}