We only use arrow_formal_parameters_error to find that the parentheses aren't
nested. E.g., (()) isn't valid. We can immediately classify those errors if
) (matching either "(" or "async(") isn't followed by =>.

The individual parameters are verified by making sure that they are valid
binding patterns with valid initializers.

Change-Id: I5f86d9896a4fa42d1ae436ced8bf2d4bd2b02c82
Reviewed-on: https://chromium-review.googlesource.com/c/1326023Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57361}