Issue 26096. Explicitly compute the result that ensures that directives are resolved in units.

pkg/analyzer/lib/src/plugin/engine_plugin.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.plugin.engine_plugin;
 
 import 'package:analyzer/plugin/task.dart';
 import 'package:analyzer/src/generated/engine.dart'
     show InternalAnalysisContext;
 import 'package:analyzer/src/generated/error.dart' show AnalysisError;
@@ skipping 206 matching lines @@
     registerExtension(
         taskId, PropagateVariableTypesInLibraryClosureTask.DESCRIPTOR);
     registerExtension(taskId, PropagateVariableTypesInLibraryTask.DESCRIPTOR);
     registerExtension(taskId, PropagateVariableTypesInUnitTask.DESCRIPTOR);
     registerExtension(taskId, PropagateVariableTypeTask.DESCRIPTOR);
     registerExtension(taskId, ReadyLibraryElement2Task.DESCRIPTOR);
     registerExtension(taskId, ReadyLibraryElement5Task.DESCRIPTOR);
     registerExtension(taskId, ReadyLibraryElement6Task.DESCRIPTOR);
     registerExtension(taskId, ReadyResolvedUnitTask.DESCRIPTOR);
     registerExtension(taskId, ResolveConstantExpressionTask.DESCRIPTOR);
+    registerExtension(taskId, ResolveDirectiveElementsTask.DESCRIPTOR);
     registerExtension(taskId, ResolveInstanceFieldsInUnitTask.DESCRIPTOR);
     registerExtension(taskId, ResolveLibraryReferencesTask.DESCRIPTOR);
     registerExtension(taskId, ResolveLibraryTask.DESCRIPTOR);
     registerExtension(taskId, ResolveLibraryTypeNamesTask.DESCRIPTOR);
     registerExtension(taskId, ResolveUnitTask.DESCRIPTOR);
     registerExtension(taskId, ResolveUnitTypeNamesTask.DESCRIPTOR);
     registerExtension(taskId, ResolveVariableReferencesTask.DESCRIPTOR);
     registerExtension(taskId, ScanDartTask.DESCRIPTOR);
     registerExtension(taskId, StrongModeVerifyUnitTask.DESCRIPTOR);
     registerExtension(taskId, VerifyUnitTask.DESCRIPTOR);
@@ skipping 62 matching lines @@
  * passed to the extension manager to populate it.
  *
  * This annotation is not used at runtime; it is used to aid in static analysis
  * of the task model during development.
  */
 class ExtensionPointId {
   final String extensionPointId;
 
   const ExtensionPointId(this.extensionPointId);
 }