main.dart

 class UserId {
  String _id;
  UserId(this._id);
  
  @override
  bool operator == (other) {
    return _id == other._id;
  }
  int get hashCode => _id.hashCode;
}

void main() {
  final map = new Map<UserId,int>();
	var a = new UserId('1');
  var b = new UserId('2');
  map[a] = 1;
  map[b] = 2;
  print(map[new UserId('1')]);
  print(map[new UserId('2')]);
}