toTimeString method
Converts the duration to a formatted time string (MM:SS).
Example:
Duration(seconds: 75).toTimeString(); // "01:15"
Implementation
String toTimeString() {
  int totalSeconds = inSeconds;
  int minutes = totalSeconds ~/ 60;
  int seconds = totalSeconds % 60;
  return '${minutes.toString().padLeft(2, '0')}:'
      '${seconds.toString().padLeft(2, '0')}';
}