freezes.dart

//from
//class Person {
//  final String name;
//  final int age;
//  
//  Person({
//    required this.name,
//    required this.age,
//  })
//}

//to
@freezed
class Person {
  const factory Person({
    required String name,
    required int age,
  }) = _Person;
}

instruction.sh

#!/bin/sh
cd /path/to/flutter/project
cd lib/
flutter pub run build_runner build --delete-conflicting-outputs

# edit code to L:13-19
flutter pub run build_runner build --delete-conflicting-outputs

# see the terminal, put something there in dart file
flutter pub run build_runner build --delete-conflicting-outputs

# copy-paste