Implement SearchEngine.searchAllSubtypes() with the analysis driver.

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.
 
 library services.search_engine;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 44 matching lines @@
 /**
  * The interface [SearchEngine] defines the behavior of objects that can be used
  * to search for various pieces of information.
  */
 abstract class SearchEngine {
   /**
    * Returns all subtypes of the given [type].
    *
    * [type] - the [ClassElement] being subtyped by the found matches.
    */
-  Future<List<SearchMatch>> searchAllSubtypes(ClassElement type);
+  Future<Set<ClassElement>> searchAllSubtypes(ClassElement type);
 
   /**
    * Returns declarations of class members with the given name.
    *
    * [name] - the name being declared by the found matches.
    */
   Future<List<SearchMatch>> searchMemberDeclarations(String name);
 
   /**
    * Returns all resolved and unresolved qualified references to the class
@@ skipping 81 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();
   }
 }