在构造函数参数列表中使用const关键字来定义常量，以确保参数值不可变。

示例代码：

@immutable
class MyImmutableClass {
  final String name;
  final int age;

  MyImmutableClass({required this.name, required this.age}) : assert(name != null && age != null);

  MyImmutableClass.fromDefault() : name = 'John', age = 18;
  
  MyImmutableClass.fromConst({String? name, int? age})
      : name = name ?? 'Tom', 
        age = age ?? 20, 
        assert(name != null && age != null);
}

void main() {
  // create an instance using default values
  var instance1 = MyImmutableClass.fromDefault();
  
  // create an instance using parameters
  var instance2 = MyImmutableClass(
    name: 'Alice',
    age: 25,
  );
  
  // create an instance using const parameters
  var instance3 = MyImmutableClass.fromConst(
    name: const String.fromEnvironment('name'),
    age: const int.fromEnvironment('age', defaultValue: 30),
  );

  print(instance1.name);  // John
  print(instance2.name);  // Alice
  print(instance3.name);  // Tom
}