Sort by multiple factors.

meeting.dart

String meeting(String s) {
  final people = s.split(';').map((String name) => Person.fromData(name)).toList();
  people.sort();
  return people.join();
}

class Person implements Comparable<Person> {
  final firstName;
  final lastName;
  
  const Person(this.firstName, this.lastName);
  
  factory Person.fromData(String data) {
    final splitData = data.toUpperCase().split(':');
    return Person(splitData.first, splitData.last);
  }
  
  int compareTo(Person other) {
    final result = lastName.compareTo(other.lastName);
    return result == 0 ? firstName.compareTo(other.firstName) : result;
  }
  
  @override
  String toString() => "($lastName, $firstName)";
}