2-3-types-4.md

void main() {  
  //a
  Map numberBook = {
    "Иван":2264865,
    "Татьяна":89523366684,
    "Олег":89452256575
  };
  //b
  print('NumberBook ${numberBook}');
  //c
  numberBook["Екатерина"] = 2359942;
  print('NumberBook ${numberBook}');
  //d i think it isn't correct task
  var keys = numberBook.keys.toList()..sort();
  var rkeys = keys.reversed;
  
  for (String key in rkeys) {
    print("$key : ${numberBook[key]}");
  }
}

Соглашусь, про мапу поправим.