Apply dart fixes (#1411)

lib/ui/collections/album/vertical_list.dart

@@ -363,7 +363,7 @@ class AlbumVerticalListWidget extends StatelessWidget {
     int collectionID,
     bool showProgressDialog,
   ) async {
-    bool result = await _collectionActions.addToCollection(
+    final bool result = await _collectionActions.addToCollection(
       context,
       collectionID,
       showProgressDialog,

lib/ui/collections/button/hidden_button.dart

@@ -80,7 +80,7 @@ class HiddenCollectionsButtonWidget extends StatelessWidget {
         if (hasAuthenticated) {
           routeToPage(
             context,
-            HiddenPage(),
+            const HiddenPage(),
           );
         }
       },

lib/ui/viewer/file/file_app_bar.dart

@@ -108,7 +108,7 @@ class FileAppBarState extends State<FileAppBar> {
         actions.add(
           Text(
             widget.file.generatedID?.toString() ?? 'null',
-            style: TextStyle(color: Colors.white),
+            style: const TextStyle(color: Colors.white),
           ),
         );
       }