join method
- String separator = ''
 
override
    Converts each element to a String and concatenates the strings.
Iterates through elements of this iterable,
converts each one to a String by calling Object.toString,
and then concatenates the strings, with the
separator string interleaved between the elements.
Example:
final planetsByMass = <double, String>{0.06: 'Mercury', 0.81: 'Venus',
  0.11: 'Mars'};
final joinedNames = planetsByMass.values.join('-'); // Mercury-Venus-Mars
Implementation
@override
String join([String separator = '']) {
  if (isEmpty) {
    return '';
  }
  final buffer = StringBuffer();
  buffer.write(elementAt(0));
  for (var i = 1; i < length; i++) {
    buffer.write(separator);
    buffer.write(elementAt(i));
  }
  return buffer.toString();
}