void main() {
  print(A()); // default
  print(A(null)); // null <-------------- false positive
  print(A('value')); // value
}

class A {
  factory A([String value]) = B;
  A._();
}

class B extends A {
  String value;
  B([this.value = 'default']) : super._();

  @override
  String toString() => value;
}