This improves the error message for []=>{}. Now rather than pointing
at ] and saying that's the unexpected token, it reports
"SyntaxError: Malformed arrow function parameter list", pointing at [.

Change-Id: I88457dc9f929a7c2b07b36ea798f11e9a7d24414
Reviewed-on: https://chromium-review.googlesource.com/c/1293369
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56847}