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

pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart

Index: pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
diff --git a/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart b/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
index 72cd49496571f0517c9bfd36d9e8bdc610f718dd..04dd6c59e9220d1eea6ce397ff65b3cae2b1cc8b 100644
--- a/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
+++ b/pkg/analysis_server/lib/src/services/search/search_engine_internal2.dart
@@ -48,8 +48,12 @@ class SearchEngineImpl2 implements SearchEngine {
 
   @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