Add a couple of new tests for DartWorkManager.

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 333 matching lines @@
           shouldSetErrors = true;
         }
       });
       if (shouldSetErrors) {
         AnalysisErrorInfo info = context.getErrors(source);
         context.getNotice(source).setErrors(info.errors, info.lineInfo);
       }
     }
   }
 
+  /**
+   * 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);
   }
 
   /**
    * Ask the [context]'s result provider for [CONTAINING_LIBRARIES].
    * Return the list of containing libraries, or `null` if unknown.
    */
   List<Source> _getLibrariesContainingPartFromResultProvider(Source part) {
     CacheEntry cacheEntry = context.getCacheEntry(part);
@@ skipping 123 matching lines @@
     }
   }
 
   bool _shouldErrorsBeComputed(Source source) =>
       context.shouldErrorsBeAnalyzed(source);
 
   static bool _isDartSource(AnalysisTarget target) {
     return target is Source && AnalysisEngine.isDartFileName(target.fullName);
   }
 }