Add a couple of new tests for DartWorkManager.

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 7de97e493c1fca73e9579a811f0e0288de004b87..f3dd0b66dbbe034df1acb3da04bbd43f8a98b183 100644
--- a/pkg/analyzer/lib/src/task/dart_work_manager.dart
+++ b/pkg/analyzer/lib/src/task/dart_work_manager.dart
@@ -351,6 +351,10 @@ class DartWorkManager implements WorkManager {
     }
   }
 
+  /**
+   * The given unit was incrementally resolved. Some of its error results might
+   * have been invalidated, so we schedule it for computing errors.
+   */
   void unitIncrementallyResolved(Source librarySource, Source unitSource) {
     librarySourceQueue.add(librarySource);
   }