How to set a function parameter's default value as an object of a class which extends an abstract class?

Let's say I have several classes which extend an abstract class. Now I want to pass a default value to a function argument, where the type of the argument is the abstract class. Dart expects a const value, and I couldn't create a const constructor for an abstract class. How can I pass a default value of the abstract class?

Sample code is as following:

class Main {

  late A objOfA;


  Main({ A nObjOfA = const B() }); // <===== Error here

}

abstract class A {

  abstract String name;
  
  abstract int id;
}

class B extends A {

  @override
  String name = "B";

  @override
  int id = 1;
}

class C extends A {

  @override
  String name = "C";

  @override
  int id = 1;
}

Here, how can I pass a default value of nObjOfA in the constructor of Main?

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