Add support to the task model for the @required annotation

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 186 matching lines @@
     registerExtension(taskId, BuildExportNamespaceTask.DESCRIPTOR);
     registerExtension(taskId, BuildLibraryElementTask.DESCRIPTOR);
     registerExtension(taskId, BuildPublicNamespaceTask.DESCRIPTOR);
     registerExtension(taskId, BuildSourceExportClosureTask.DESCRIPTOR);
     registerExtension(taskId, BuildTypeProviderTask.DESCRIPTOR);
     registerExtension(taskId, ComputeConstantDependenciesTask.DESCRIPTOR);
     registerExtension(taskId, ComputeConstantValueTask.DESCRIPTOR);
     registerExtension(
         taskId, ComputeInferableStaticVariableDependenciesTask.DESCRIPTOR);
     registerExtension(taskId, ComputeLibraryCycleTask.DESCRIPTOR);
+    registerExtension(taskId, ComputeRequiredConstantsTask.DESCRIPTOR);
     registerExtension(
         taskId, ComputePropagableVariableDependenciesTask.DESCRIPTOR);
     registerExtension(taskId, ContainingLibrariesTask.DESCRIPTOR);
     registerExtension(taskId, DartErrorsTask.DESCRIPTOR);
     registerExtension(taskId, EvaluateUnitConstantsTask.DESCRIPTOR);
     registerExtension(taskId, GatherUsedImportedElementsTask.DESCRIPTOR);
     registerExtension(taskId, GatherUsedLocalElementsTask.DESCRIPTOR);
     registerExtension(taskId, GenerateHintsTask.DESCRIPTOR);
     registerExtension(taskId, GenerateLintsTask.DESCRIPTOR);
     registerExtension(taskId, InferInstanceMembersInUnitTask.DESCRIPTOR);
@@ skipping 56 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);
 }