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