ApiExporter should be final (#8262)

Author: jonasfj

app/lib/package/api_export/api_exporter.dart

@@ -32,7 +32,7 @@ ApiExporter? get apiExporter => ss.lookup(#_apiExporter) as ApiExporter?;
 
 const _concurrency = 50;
 
-class ApiExporter {
+final class ApiExporter {
   final DatastoreDB _db;
 
   final ExportedApi _api;