Add SearchEngine.membersOfSubtypes().

pkg/analysis_server/lib/src/services/search/search_engine.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
@@ skipping 38 matching lines @@
   @override
   String toString() => name;
 }
 
 /**
  * The interface [SearchEngine] defines the behavior of objects that can be used
  * to search for various pieces of information.
  */
 abstract class SearchEngine {
   /**
+   * If the [type] has subtypes, return the set of names of members which these
+   * subtypes declare, possibly empty.  If the [type] does not have subtypes,
+   * return `null`.
+   */
+  Future<Set<String>> membersOfSubtypes(ClassElement type);
+
+  /**
    * Returns all subtypes of the given [type].
    *
    * [type] - the [ClassElement] being subtyped by the found matches.
    */
   Future<Set<ClassElement>> searchAllSubtypes(ClassElement type);
 
   /**
    * Returns declarations of class members with the given name.
    *
    * [name] - the name being declared by the found matches.
@@ skipping 86 matching lines @@
   /**
    * Return elements of [matches] which has not-null elements.
    *
    * When [SearchMatch.element] is not `null` we cache its value, so it cannot
    * become `null` later.
    */
   static List<SearchMatch> withNotNullElement(List<SearchMatch> matches) {
     return matches.where((match) => match.element != null).toList();
   }
 }