Implement search for direct subtypes and enable `search.getTypeHierarchy` request.

pkg/analysis_server/lib/src/services/search/search_engine_internal2.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.
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/dart/analysis/search.dart';
@@ skipping 30 matching lines @@
     List<SearchResult> allResults = [];
     for (AnalysisDriver driver in _drivers) {
       List<SearchResult> results = await driver.search.references(element);
       allResults.addAll(results);
     }
     return allResults.map(_SearchMatch.forSearchResult).toList();
   }
 
   @override
   Future<List<SearchMatch>> searchSubtypes(ClassElement type) async {
-    // TODO(scheglov) implement
-    return [];
+    List<SearchResult> allResults = [];
+    for (AnalysisDriver driver in _drivers) {
+      List<SearchResult> results = await driver.search.subTypes(type);
+      allResults.addAll(results);
+    }
+    return allResults.map(_SearchMatch.forSearchResult).toList();
   }
 
   @override
   Future<List<SearchMatch>> searchTopLevelDeclarations(String pattern) async {
     // TODO(scheglov) implement
     return [];
   }
 }
 
 class _SearchMatch implements SearchMatch {
@@ skipping 77 matching lines @@
     }
     if (kind == SearchResultKind.WRITE) {
       return MatchKind.WRITE;
     }
     if (kind == SearchResultKind.INVOCATION) {
       return MatchKind.INVOCATION;
     }
     return MatchKind.REFERENCE;
   }
 }