Named parameters can't start with underscores whether they are optional or required.
Change-Id: Iaaeab80f74a691b4981148f06412a60ca115f73e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337883
Auto-Submit: Parker Lougheed <parlough@gmail.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
