function getSampleMapSet() {
  var mapSet = new lf.structs.MapSet();
  mapSet.set(10, 11);
  mapSet.set(10, 12);
  mapSet.set(20, 21);
  mapSet.set(20, 25);
  mapSet.set(30, 39);
  return mapSet;
}
Creates a pre-populated MapSet for testing.
@return {!lf.structs.MapSet<number, number>}