When extend with an real class (not Object), got the error "The class 'TumblrPostEntity' doesn't have an unnamed constructor.".

[jaggerwang]

Problem

As the title. All the examples are using class extends from Object, so I have no reference.

The class 'TumblrPostEntity' doesn't have an unnamed constructor.

Code

import 'package:meta/meta.dart';
import 'package:json_annotation/json_annotation.dart';

part 'tumblr_post.g.dart';

@JsonSerializable()
@immutable
class TumblrPostEntity extends Object with _$TumblrPostEntitySerializerMixin {
  final int id;
  final String url;
  final String shortUrl;
  final String type;
  final String format;
  final List<String> tags;
  final String reblogKey;
  final _TumblrPostSource source;
  final bool isLiked;
  final String creator;
  final DateTime postAt;

  TumblrPostEntity({
    this.id = 0,
    this.url = '',
    this.shortUrl = '',
    this.type = '',
    this.format = '',
    this.tags = const [],
    this.reblogKey = '',
    source,
    this.isLiked = false,
    this.creator = '',
    postAt,
  })  : this.source = source ?? _TumblrPostSource(),
        this.postAt = postAt ?? DateTime.fromMillisecondsSinceEpoch(0);

  factory TumblrPostEntity.fromJson(Map<String, dynamic> json) =>
      _$TumblrPostEntityFromJson(json);

  @override
  String toString() => 'TumblrPostEntity${toJson()}';
}

@JsonSerializable()
@immutable
class _TumblrPostSource extends Object with _$_TumblrPostSourceSerializerMixin {
  final String title;
  final String url;

  _TumblrPostSource({
    this.title = '',
    this.url = '',
  });

  factory _TumblrPostSource.fromJson(Map<String, dynamic> json) =>
      _$_TumblrPostSourceFromJson(json);

  @override
  String toString() => '_TumblrPostSource${toJson()}';
}

@JsonSerializable()
@immutable
class TumblrTextPostEntity extends TumblrPostEntity
    with _$TumblrTextPostEntitySerializerMixin {
  final String title;
  final String body;

  TumblrTextPostEntity({
    id = 0,
    url = '',
    shortUrl = '',
    type = '',
    format = '',
    tags = const [],
    reblogKey = '',
    source = 0,
    isLiked = false,
    creator = '',
    postAt,
    this.title = '',
    this.body = '',
  }) : super(
          id: id,
          url: url,
          shortUrl: shortUrl,
          type: type,
          format: format,
          tags: tags,
          reblogKey: reblogKey,
          source: source,
          isLiked: isLiked,
          creator: creator,
          postAt: postAt,
        );

  factory TumblrTextPostEntity.fromJson(Map<String, dynamic> json) =>
      _$TumblrTextPostEntityFromJson(json);

  @override
  String toString() => 'TumblrTextPostEntity${toJson()}';
}

[jaggerwang]

Merge to another.