private_optional_parameter

Named parameters can't start with an underscore.

The analyzer produces this diagnostic when the name of a named parameter starts with an underscore.

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

class C {
  void m({int _x = 0}) {}
}

Rename the parameter so that it doesn't start with an underscore:

class C {
  void m({int x = 0}) {}
}