Feature: Refactor class modifier when dart 3 migration is all set.

[ntut-xuan]

Feature Description

The issue is used to memorize the TODO.

We are trying to migrate the project from dart 2 into the dart 3 now, the current project dart sdk version support < 3.0.0, it shows that we can't use the class modifier support in dart 3.

We expected that when the migration is all set, it should add the class modifier to the data transfer object.