Issue 24639. Check the correct object for null.

pkg/analyzer/lib/src/task/dart_work_manager.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.task.dart_work_manager;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/engine.dart'
@@ skipping 382 matching lines @@
         Source library = target.library;
         if (context.exists(library)) {
           librarySourceQueue.add(library);
         }
       }
     }
     // Invalidate targets and values.
     unitTargets.forEach(partition.remove);
     for (Source dartSource in dartSources) {
       CacheEntry entry = partition.get(dartSource);
-      if (dartSource != null) {
+      if (entry != null) {
         // TODO(scheglov) we invalidate too much.
         // Would be nice to invalidate just URLs resolution.
         entry.setState(PARSED_UNIT, CacheState.INVALID);
         entry.setState(IMPORTED_LIBRARIES, CacheState.INVALID);
         entry.setState(EXPLICITLY_IMPORTED_LIBRARIES, CacheState.INVALID);
         entry.setState(EXPORTED_LIBRARIES, CacheState.INVALID);
         entry.setState(INCLUDED_PARTS, CacheState.INVALID);
       }
     }
   }
@@ skipping 53 matching lines @@
     }
   }
 
   bool _shouldErrorsBeComputed(Source source) =>
       context.shouldErrorsBeAnalyzed(source, null);
 
   static bool _isDartSource(AnalysisTarget target) {
     return target is Source && AnalysisEngine.isDartFileName(target.fullName);
   }
 }