sortedByCompare<K>  method 
    Null safety
- K keyOf(
- T element
 
 - Comparator<
K> compare 
Creates a sorted list of the elements of the iterable.
The elements are ordered by the compare Comparator of the
property keyOf of the element.
Implementation
List<T> sortedByCompare<K>(
    K Function(T element) keyOf, Comparator<K> compare) {
  var elements = [...this];
  mergeSortBy<T, K>(elements, keyOf, compare);
  return elements;
}