input: Write a python function to manage flight schedules by adding and retrieving flight information.
  assert get_flight_info('Flight001') == {'source': 'New York', 'destination': 'Los Angeles', 'departure_time': '06:00 AM', 'arrival_time': '09:00 AM'}
  assert get_flight_info('Flight999') == 'Flight does not exist.'
  assert add_flight('Flight002', 'Chicago', 'San Francisco', '08:00 AM', '11:00 AM') == None
  assert get_flight_info('Flight002') == {'source': 'Chicago', 'destination': 'San Francisco', 'departure_time': '08:00 AM', 'arrival_time': '11:00 AM'}
output: flight_schedule = {}
  
  def add_flight(flight_id, source, destination, departure_time, arrival_time):
      flight_schedule[flight_id] = {
          'source': source,
          'destination': destination,
          'departure_time': departure_time,
          'arrival_time': arrival_time
      }
  
  def get_flight_info(flight_id):
      return flight_schedule.get(flight_id, 'Flight does not exist.')