`Add final initializing formal required named parameters` fix adds private fields without wrapping

[FMorschel]

Repro:

class A {
  A({int? other});

  final int _var;
}

Becomes:

class A {
  A({int? other, required this._var});

  final int _var;
}

FYI @bwilkerson

I'll take a look at this.

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/459502

Hopefully, we can land this soon to make the work on declaring constructors easier.