سلام آرین جان مشکلو حل کردم نکته اول اینکه متد LetsDrive باید قبل استفاده ازش define بشه . من کد ویرایش شده رو برات میذارم :

void main() {
  void letsDrive(Car car2){
  car2.engine_start() ;
//   car2.accelarate() ;
}
letsDrive(bmwsedanCar(03 , 'bmw' , 'sedan' , 'Germany')) ;
//Car car = new Car(01 , 'BMW' , 'M4' , 'coupe' , 330 , 'Germany') ;
//Car car = new Car.German(02 , 'Mercedes' , 'CLK' , 'race_car' , 350) ;
bmwsedanCar car2 = new bmwsedanCar(03 , 'bmw' , 'sedan' , 'Germany') ;
//car.engine_start() ;
car2.engine_start() ;
}
  class Car{
int id ;
String company ;
String model ;
String type ;
int maxspeed ;
String created_at ;
Car(this.id , this.company , this.model ,this.type , this.maxspeed , this.created_at){
print('new car created') ;
}
Car.German(this.id , this.company , this.model , this.type , this.maxspeed):created_at='Germany' ;
void engine_start(){
  print('the Engine of $company $model started') ;
}
void engine_stop(){
  print('the Engine of $company $model stopped') ;
}
  }
  // ers vary kardan 1 class az 1 class dige (Inheritance) ---------
  class bmwsedanCar extends Car {
bmwsedanCar
(int id,String company,String type,String created_at):super(03,'bmw','535','sedan',250,created_at){
  print('new sedan car of bmw created') ;
}
@override
void engine_start(){
  super.engine_start() ;
  print('sedan bmw started') ;
}
@override
void engine_stop(){
  super.engine_stop() ;
  print('sedan bmw stopped') ;
}
  }