fixup! fixup! TW-1586: add testing to guarantee that only 1 file is d…

…ownloading at once

lib/utils/task_queue/worker_queue.dart

@@ -8,7 +8,7 @@ typedef OnTaskCompleted = void Function(String? taskId);
 
 abstract class WorkerQueue {
   final Queue<Task> queue = Queue<Task>();
-  Completer? completer;
+  Completer? _completer;
 
   String get workerName;
 
@@ -21,13 +21,13 @@ abstract class WorkerQueue {
   }
 
   Future _processTask() async {
-    if (completer != null) {
-      return completer!.future;
+    if (_completer != null) {
+      return _completer!.future;
     }
     Logs().i(
       'WorkerQueue<$workerName>::addTask(): QUEUE is ready',
     );
-    completer = Completer();
+    _completer = Completer();
     if (queue.isNotEmpty) {
       final firstTask = queue.removeFirst();
       Logs().i('WorkerQueue<$workerName>::_processTask(): ${firstTask.id}');
@@ -37,14 +37,14 @@ abstract class WorkerQueue {
           .catchError(_handleTaskExecuteError)
           .whenComplete(() => firstTask.onTaskCompleted?.call());
     } else {
-      completer?.complete();
+      _completer?.complete();
     }
-    return completer!.future;
+    return _completer!.future;
   }
 
   void _handleTaskExecuteCompleted(dynamic value) {
     Logs().i('WorkerQueue<$workerName>::_handleTaskExecuteCompleted(): $value');
-    completer?.complete(value);
+    _completer?.complete(value);
     _releaseCompleter();
     if (queue.isNotEmpty) {
       _processTask();
@@ -53,15 +53,15 @@ abstract class WorkerQueue {
 
   void _handleTaskExecuteError(error) {
     Logs().i('WorkerQueue<$workerName>::_handleTaskExecuteError(): $error');
-    completer?.complete(error);
+    _completer?.complete(error);
     _releaseCompleter();
     if (queue.isNotEmpty) {
       _processTask();
     }
   }
 
   void _releaseCompleter() {
-    completer = null;
+    _completer = null;
   }
 
   Future release() async {