anchor.diff:          if (!stringifyJsonValue(customJson)) {
             throw new JsonUnsupportedObjectError(object);
           }
  -        _seen.remove(object);
  +        _removeSeen(object);
         } catch (e) {
           throw new JsonUnsupportedObjectError(object, cause: e);
         }
  
positive.code:         stringifyValue(m[key]);
        }
        _sink.write('}');
        _seen.remove(object);
        return true;
      } else {
        return false;
      }
    }
  }
  
negative.code:   }
  
    void checkCycle(object) {
      for (int i = 0; i < _seen.length; i++) {
        if (identical(object, _seen[i])) {
          throw new JsonCyclicError(object);
        }
      }
      _seen.add(object);
    }
  
    void stringifyValue(object) {
  