private_setter

The setter '{0}' is private and can't be accessed outside the library that declares it.

The analyzer produces this diagnostic when a private setter is used in a library where it isn't visible.

Given a file a.dart that contains the following:

class A {
  static int _f = 0;
}

The following code produces this diagnostic because it references the private setter _f even though the setter isn't visible:

import 'a.dart';

void f() {
  A._f = 0;
}

If you're able to make the setter public, then do so:

class A {
  static int f = 0;
}

If you aren't able to make the setter public, then find a different way to implement the code.