Implement SearchEngine.searchAllSubtypes() with the analysis driver.

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

 // Copyright (c) 2016, 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 analysis_server.src.services.search.search_engine_internal;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/correction/source_range.dart';
 import 'package:analysis_server/src/services/index/index.dart';
@@ skipping 12 matching lines @@
 
 /**
  * A [SearchEngine] implementation.
  */
 class SearchEngineImpl implements SearchEngine {
   final Index _index;
 
   SearchEngineImpl(this._index);
 
   @override
-  Future<List<SearchMatch>> searchAllSubtypes(ClassElement type) async {
+  Future<Set<ClassElement>> searchAllSubtypes(ClassElement type) async {
     List<SearchMatch> matches = <SearchMatch>[];
     await _addMatches(
         matches, type, IndexRelationKind.IS_ANCESTOR_OF, MatchKind.DECLARATION);
-    return matches;
+    return matches.map((match) => match.element as ClassElement).toSet();
   }
 
   @override
   Future<List<SearchMatch>> searchMemberDeclarations(String name) {
     String pattern = '^$name\$';
     return _searchDefinedNames(pattern, IndexNameKind.classMember);
   }
 
   @override
   Future<List<SearchMatch>> searchMemberReferences(String name) async {
@@ skipping 527 matching lines @@
       _addMatch(node, kind);
     }
   }
 
   void _addMatch(AstNode node, MatchKind kind) {
     bool isQualified = node.parent is Label;
     matches.add(new SearchMatchImpl(context, libraryUri, unitUri, kind,
         rangeNode(node), true, isQualified));
   }
 }