anchor.diff:           num n;
            A a;
            B b;
  -         C c;
            o = y;
            i = /*info:Unbox*/y;
            d = /*info:Unbox*/y;
            n = /*info:DownCast*/y;
            a = /*info:DownCast*/y;
            b = /*info:DownCast*/y;
  -         c = /*info:DownCast*/y;
         }
      '''
       });
  
positive.code: 
        class A {}
        class B extends A {}
        class C extends A {}
  
        void main() {
           dynamic y;
  
negative.code:          n = /*severe:StaticTypeError*/a;
           a = a;
           b = /*info:DownCast*/a;
        }
     '''
      });
  