private_named_non_field_parameter

Named parameters that don't refer to instance variables can't start with underscore.

The analyzer produces this diagnostic when a named parameter starts with an underscore, unless it's an initializing formal or field parameter.

The following code produces this diagnostic because the named parameter _x starts with an underscore:

class C {
  C({int _x = 0});
}

If the parameter is intended to refer to a field, then add the missing field:

class C {
  final int _x;
  C({this._x = 0});
  int get x => _x;
}

If the parameter isn't intended to refer to a field, then remove the underscore:

class C {
  C({int x = 0});
}