anchor.diff:    final _Reviver _reviver;
     _ReviverJsonCodec(this._reviver);
   
  -  Object decode(String source, {reviver(var key, var value)}) {
  +  dynamic decode(String source, {reviver(var key, var value)}) {
       if (reviver == null) reviver = _reviver;
       return new JsonDecoder(reviver).convert(source);
     }
  
positive.code:    *
     * The default [reviver] (when not provided) is the identity function.
     */
    Object decode(String source, {reviver(var key, var value)}) {
      if (reviver == null) return decoder.convert(source);
      return new JsonDecoder(reviver).convert(source);
    }
  
negative.code:    * If [toEncodable] is omitted, it defaults to calling `.toJson()` on the
     * unencodable object.
     */
    String encode(Object value, {toEncodable(var object)}) {
      if (toEncodable == null) return encoder.convert(value);
      return new JsonEncoder(toEncodable).convert(value);
    }
  