Firestore keyword working in the code: Flutter

Question

In the below code Firestore.instance.collection keyword gives an error. I can not understand why this error occurs help to resolve this error.

 void _fetchMarkersFromDb() {
        // TODO: improve this
        print('_fetchMarkersFromDb() called');
        ***Firestore***.instance.collection('markers').getDocuments().then((docs) async {
          final docLength = docs.documents.length;
          final clients = List(docLength);
          for (int i = 0; i < docLength; i++) {
            clients[i] = docs.documents[i];
          }
          if (!isFirstCycle && isMyMarkerFetched) {
            currentLocation = await Geolocator().getCurrentPosition();
          }
          _populateMarkers(clients);
        });
      } // fetches markers from firestore