How to update some old Flutter code to latest release

I have a Flutter app that was started and abandoned about four years ago. The project has resurfaced and I have been fixing the deprecated items that arose due to the updates made to Flutter in that time period. I have all but one area ready to begin testing, and that is reading from a Firebase Realtime Database into a List. The following is the code that has the error:

  return await dbReference
  .orderByChild(AppDatabase._speciesCommonName)
  .startAt(prefix)
  .endAt('$prefixuf8ff')
  .once()
  .then((DataSnapshot dataSnapshot) {
//
// Make sure we have data
//
if (dataSnapshot.value != null) {
  dataSnapshot.value.forEach((key, value) {
    String dbKey = key;
    String commonName = value[AppDatabase._speciesCommonName];
    Species obj = Species(dbKey: dbKey, commonName: commonName);
    list.add(obj);
  });

The error I get is:

The argument type ‘List Function(DataSnapshot)’ can’t be assigned to the parameter type ‘FutureOr<List> Function(DatabaseEvent)’.

I have searched and studied this code and the error, however, having been away from Flutter for those years I am failing to figure out the resolution.

A second error is also reported in the above code, however, I am hoping if the inital error is fixed so will this one:

The method ‘forEach’ isn’t defined for the type ‘Object’.

Any input would be greatly appreciated.