anchor.diff:          if (priority.index > bestPriority.index) {
             bestDriver = driver;
             bestPriority = priority;
  +        }
  +      }
  +
  +      if (outOfBandWorker != null) {
  +        var workerPriority = outOfBandWorker.workPriority;
  +        if (workerPriority != AnalysisDriverPriority.nothing) {
  +          if (workerPriority.index > bestPriority.index) {
  +            await outOfBandWorker.performWork();
  +            _hasWork.notify();
  +            continue;
  +          }
           }
         }
   
  
positive.code: 
    bool _started = false;
  
    AnalysisDriverScheduler(this._logger, {this.driverWatcher});
  
    /**
  
negative.code:   ExceptionResult(this.path, this.exception, this.contextKey);
  }
  
  /// Worker in [AnalysisDriverScheduler].
  abstract class SchedulerWorker {
    /// Return the priority of work that this worker needs to perform.
    AnalysisDriverPriority get workPriority;
  
    /// Perform a single chunk of work.
    Future<void> performWork();
  }
  
  /**
   * Task that discovers all files that are available to the driver, and makes
   * them known.
  