“var teclas = snap.value.keys;” Código de respuesta

var teclas = snap.value.keys;

@override
  void initState() {
  super.initState();
    final ref = FirebaseDatabase.instance.ref().child('employee');

    ref.once().then((snap){
  //TO retrieve all the documents from collection
  dynamic keys = snap.snapshot.value;
  //To get only the document IDs of all documents.
  dynamic KEYS = keys.keys;

  dynamic data = snap.snapshot.value;
        for (var key in KEYS){
          Employee d = new Employee(
            id : data [key]['id'],
            name : data[key]['name'],
            email : data[key]['email'],
            emp_id: data[key]['emp_id']
          );
          myAllData.add(d);
        }


      setState(() {
       print('Length : ${myAllData.length}');
      });
    });