anchor.diff:      if (val != null) {
         return int.parse(val, onError: (_) => null);
       }
  -    return val;
  +    return int.parse(val);
     }
   
     @override
  
positive.code: 
    @override
    void forEach(void f(String name, List<String> values)) {
      var forEachFunc = (name) {
        var values = this[name];
        if (values != null && values.isNotEmpty) {
          f(name, values);
        }
      };
  
      [
        HttpHeaders.ACCEPT_CHARSET,
  
negative.code:   }
  
    @override
    void clear() {
      _headers.clear();
    }
  
    @override
    void set(String name, Object value) {
      removeAll(name);
      add(name, value);
  