anchor.diff:    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.
  
positive.code:         if (priority.index > bestPriority.index) {
            bestDriver = driver;
            bestPriority = priority;
          }
        }
  
  
negative.code: 
    bool _started = false;
  
    /**
     * The optional worker that is invoked when its work priority is higher
     * than work priorities in drivers.
     *
     * Don't use outside of Analyzer and Analysis Server.
     */
    SchedulerWorker outOfBandWorker;
  
    AnalysisDriverScheduler(this._logger, {this.driverWatcher});
  
    /**
  