How to write factory constructor for abstract class, then use factory constructor in classes where dependents of abstract class is passed as template

Question

I am trying to write a generic class Repository which takes a template T and build T from a Json (Map<String, dynamic>). For that I wrote an abstract class:

abstract class JsonSerializable {
  String toJson();
  JsonSerializable.fromJson(Map<String, dynamic> json);
}

Then I created an item/object class which extends the JsonSerializable class. This is supposed to be done by any class which is supposed to be passed as a Template in the repository class' methods. The item/object class is:

class Item extends JsonSerializable {
  Item.fromJson(super.json) : super.fromJson();

  @override
  String toJson() {
    // TODO: implement toJson
    throw UnimplementedError();
  }
}

The Repository class which takes the template is as follows:

class Repository<T extends JsonSerializable> {
  const Repository();

  Future<T> getIdFromJson(Map<String, dynamic> json) async {
    return T.fromJson(json)['id'];
  }
}

But I am getting the compiler error:

The method 'fromJson' isn't defined for the type 'Type'.
Try correcting the name to the name of an existing method, or defining a method named 'fromJson'.

I also tried to change extends to implements in Item class like:

class Item implements JsonSerializable {
  const Item();

  @override
  factory Item.fromJson(Map<String, dynamic> json) {
    return const Item();
  }

  @override
  String toJson() {
    throw UnimplementedError();
  }
}

But still getting the same error.

What should I change to achieve this functionality. I am relatively new to Dart/Flutter. Please help.