How to import part of in dart?

Question

I'm using built_value, I have created a class that has part

part 'custom_value.g.dart';
    
    abstract class CustomValue implements Built<CustomValue, CustomValueBuilder> {...some code}

it's the generated class with part of:

part of 'custom_value.dart';

class _$CustomValue extends CustomValue {...some code}

and now in the class where I have imported "import './custom_value.dart';" CustomValue it doesn't see the inheriting object _$CustomValue

error message:

Undefined name '_$CustomValue'. Try correcting the name to one that is defined, or defining the name.

can it be solved somehow? i need this class for serialization