`Add final initializing formal (required named) parameters` invalid code and missing cases

[FMorschel]

Repro:

class C {
  C();
  C._();
  C.named();

  final int field;
}

1. Only the first constructor shows Add final initializing formal (required named) parameters (maybe it doesn't know how to handle named constructors somehow?)
2. The required, named version doesn't add the initial { if it isn't present, leading to invalid code

CC @bwilkerson