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