New test262 tests bring up a couple cases with async arrow functions
that V8 didn't seem to handle properly; this patch makes those cases
errors:
- async (...x,) => y -- Rest parameter must be last formal parameter
- async (...x = z) => y -- No default value for rest parameter
- async (...x, y) => z -- Rest parameter must be last formal parameter

Bug: v8:4483, v8:5051
Change-Id: I024d9ba0c854e8e5e75283df2ee53127b1be090d
Reviewed-on: https://chromium-review.googlesource.com/496057
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#45116}