如何"flutter pub run build_runner build“用类在Flutter中有字段时间戳？

dev_dependencies:
  build_runner: ^2.1.4
  json_serializable: ^5.0.2

import 'package:json_annotation/json_annotation.dart';

@JsonSerializable()
class MyClass {
  @JsonKey(fromJson: fromTimestamp, toJson: toTimestamp)
  DateTime timestamp;

  MyClass(this.timestamp);

  factory MyClass.fromJson(Map<String, dynamic> json) => _$MyClassFromJson(json);
  Map<String, dynamic> toJson() => _$MyClassToJson(this);

  static DateTime fromTimestamp(int timestamp) => DateTime.fromMillisecondsSinceEpoch(timestamp);
  static int toTimestamp(DateTime dateTime) => dateTime.millisecondsSinceEpoch;
}

flutter pub run build_runner build