invalid_use_of_protected_member

The member '{0}' can only be used within instance members of subclasses of '{1}'.

The analyzer produces this diagnostic when a getter, setter, field, or method that has been annotated with @protected is referenced anywhere other than in the library in which it is declared or in a subclass of the class in which it is declared.

Given a file a.dart that contains

import 'package:meta/meta.dart';

class A {
  @protected
  void a() {}
}

The following code produces this diagnostic because the method a is being invoked in code that isn't in a subclass of A:

import 'a.dart';

void b(A a) {
  a.a();
}

If it's reasonable for the member to be marked as @protected, then remove the reference to the protected member, replacing it with some equivalent code.

If it isn't reasonable for the member to be marked as @protected, and you have the ability to do so, remove the annotation.