make fields which are not re-assigned as final

lib/ui/password_reentry_page.dart

@@ -18,7 +18,7 @@ class PasswordReentryPage extends StatefulWidget {
 
 class _PasswordReentryPageState extends State<PasswordReentryPage> {
   final _passwordController = TextEditingController();
-  FocusNode _passwordFocusNode = FocusNode();
+  final FocusNode _passwordFocusNode = FocusNode();
   bool _passwordInFocus = false;
   bool _passwordVisible = false;
 

lib/ui/progress_dialog.dart

@@ -180,7 +180,7 @@ class ProgressDialog {
 
 // ignore: must_be_immutable
 class _Body extends StatefulWidget {
-  _BodyState _dialog = _BodyState();
+  final _BodyState _dialog = _BodyState();
 
   update() {
     _dialog.update();

lib/ui/shared_collections_gallery.dart

@@ -31,7 +31,7 @@ class SharedCollectionGallery extends StatefulWidget {
 
 class _SharedCollectionGalleryState extends State<SharedCollectionGallery>
     with AutomaticKeepAliveClientMixin {
-  Logger _logger = Logger("SharedCollectionGallery");
+  final Logger _logger = Logger("SharedCollectionGallery");
   StreamSubscription<LocalPhotosUpdatedEvent> _localFilesSubscription;
   StreamSubscription<CollectionUpdatedEvent> _collectionUpdatesSubscription;
   StreamSubscription<UserLoggedOutEvent> _loggedOutEvent;