anchor.diff:    }
   
     bool _locked = false;
  -  final _outstandingRequests = Queue<_LockRequest>();
  +  final _outstandingRequests = Queue<Completer<void>>();
   }
  
positive.code: 
  import 'dart:async';
  import 'dart:collection';
  
  typedef _LockRequest = Completer<void>;
  
  /// Used to protect global state accessed in blocks containing calls to
  /// asynchronous methods.
  
negative.code:       _locked = true;
        return;
      }
      final request = Completer<void>();
      _outstandingRequests.add(request);
      await request.future;
    }
  