anchor.diff:        _locked = true;
         return;
       }
  -    final request = _LockRequest();
  +    final request = Completer<void>();
       _outstandingRequests.add(request);
       await request.future;
     }
  
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:   }
  
    bool _locked = false;
    final _outstandingRequests = Queue<Completer<void>>();
  }
  