An AnalysisTarget is a priority one if its Source is in prioritySources.

pkg/analyzer/lib/src/context/cache.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.context.cache;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 1211 matching lines @@
     }
     CacheFlushManager manager = _flushManagerMap[policy];
     if (manager == null) {
       manager = new CacheFlushManager(policy, _isPriorityAnalysisTarget);
       _flushManagerMap[policy] = manager;
     }
     return manager;
   }
 
   bool _isPriorityAnalysisTarget(AnalysisTarget target) {
-    return context.priorityTargets.contains(target);
+    Source source = target.source;
+    return source != null && context.prioritySources.contains(source);
   }
 
   /**
    * If the given [target] is a [Source], remove it from the list of [sources].
    */
   void _removeIfSource(AnalysisTarget target) {
     if (target is Source) {
       sources.remove(target);
       String path = target.fullName;
       List<Source> pathSources = pathToSource[path];
@@ skipping 283 matching lines @@
   void resultAccessed(TargetedResult result) {}
 
   @override
   List<TargetedResult> resultStored(TargetedResult newResult, newValue) {
     return TargetedResult.EMPTY_LIST;
   }
 
   @override
   void targetRemoved(AnalysisTarget target) {}
 }