invalid_non_virtual_annotation

The annotation '@nonVirtual' can only be applied to a concrete instance member.

The analyzer produces this diagnostic when the nonVirtual annotation is found on a declaration other than a member of a class, mixin, or enum, or if the member isn't a concrete instance member.

The following code produces this diagnostic because the method m is an abstract method:

import 'package:meta/meta.dart';
​
abstract class C {
  @nonVirtual
  void m();
}

If the member is intended to be a concrete instance member, then make it so:

import 'package:meta/meta.dart';
​
abstract class C {
  @nonVirtual
  void m() {}
}