How to properly implement repository in flutter/dart - flutter

Question

Being new to flutter and dart, I am looking for ways to structure my projects. I have found that the Repository pattern made its way to dart (I have a strong Java background), but I have found that there's not the necessary tools yet to properly implement a repository.
How I would normally approach this is to first create the simple straightforward methods. I ended up with the following dart code:
abstract class CategoryRepository {
Future<List<Category>> getCategories();
Future<Category?> getCategoryById(String id);
}
All is good. But now imagine I have a PostRepository, and I only want to return posts of a specific category? I am missing a standardized way in flutter how to add conditions to my repository select methods. I cannot believe nobody has written something for this purpose before so I am doubting whether I am on the right track here. Maybe my Java background is throwing me off.
In Java (specifically Spring JPA) I am used to specifications, predicates and criteria as documented here.
In .NET you can implement select methods of a repository pattern with the Expression class which allows you to use LINQ to add select criteria as documented here.
For Flutter or Dart, what would be the equivalent? Or am I on the wrong track?
I have found the flutter_repository package, but it is deprecated (without a reason). Looking at the documentation it indeed had Specifications which are used to add conditions. This would be exactly what I expect/need, but again it's deprecated.
This is where I cannot find the information publicly anymore and am questioning "how to properly implement repository in flutter/dart", hence the reason for posting this.