x.dart

abstract class B {
}

class C extends B {
  bool c;
}

abstract class Base<T extends B> {
  T get f => null;
}

abstract class Mixin extends Base<B> {
}

class X extends Base<C> with Mixin {
  bool get c => f.c;
}

// Fasta reports:
//   flu.dart:19:19: Error: The getter 'c' isn't defined for the class '#lib1::B'.
//   Try correcting the name to the name of an existing getter, or defining a getter or field named 'c'.
//     bool get c => f.c;
//                     ^