Wildcards Update - Removed no defaults needed for unnamed optional wildcard parameters

[kallentu]

We revoked allowing unnamed optional wildcard parameters to not have a default value.
So the following is an error:

int foo([int _]) => 2;
//           ^

Not sure if there were tests made for this one spec change, but we reverted this from the spec and I want to make sure that the co19 tests aren't testing for this behaviour. Feel free to close if this is already the case.

Issue of this change: dart-lang/language#3807

cc. @sgrekhov @eernstg

[sgrekhov]

Fixed by #2730