Issue 24639. Check the correct object for null.

pkg/analyzer/lib/src/task/dart_work_manager.dart

Index: pkg/analyzer/lib/src/task/dart_work_manager.dart
diff --git a/pkg/analyzer/lib/src/task/dart_work_manager.dart b/pkg/analyzer/lib/src/task/dart_work_manager.dart
index 61f92e5568edc1ba031b160aa737c809cd16a230..62a25ea9a89375d7e89887a7a62a2de744c7a4ba 100644
--- a/pkg/analyzer/lib/src/task/dart_work_manager.dart
+++ b/pkg/analyzer/lib/src/task/dart_work_manager.dart
@@ -400,7 +400,7 @@ class DartWorkManager implements WorkManager {
     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);