Reason for revert:
needed for properly reverting f3ae5ccf57
Original issue's description:
> Thread maybe-assigned through the bytecodes.
>
> This introduces LoadImmutableContextSlot and LoadImmutableCurrentContextSlot
> bytecodes, which are emitted when reading from never-assigned context slot.
>
> There is a subtlety here: the slot are not immutable, the meaning is
> actually undefined-or-hole-or-immutable.
>
> Review-Url: https://codereview.chromium.org/2655733003
> Cr-Commit-Position: refs/heads/master@{#43000}
> Committed: 17c2dd3886TBR=rmcilroy@chromium.org,bmeurer@chromium.org,neis@chromium.org,jarin@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
Review-Url: https://codereview.chromium.org/2680923003
Cr-Commit-Position: refs/heads/master@{#43011}
ece4e54a31