[Wildcard Variables] `private_optional_parameter` support

[pq]

Today,

void f({String? _}) { }

produces a private_optional_parameter diagnostic. I expect this should be legal with wildcards and no longer produce this warning?

EDIT: Similarly for:

void f({required String _}) { }

/cc @kallentu @lrhn @bwilkerson

[pq]

Ah, sorry. This is covered in the spec:

https://github.com/dart-lang/language/blob/main/working/wildcards/feature-specification.md#declarations-that-are-capable-of-declaring-a-wildcard

Sorry for the noise!