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

pkg/analyzer/lib/src/dart/analysis/search.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:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/visitor.dart';
@@ skipping 56 matching lines @@
       return _searchReferences_Parameter(element);
     } else if (kind == ElementKind.PREFIX) {
       return _searchReferences_Prefix(element);
     } else if (kind == ElementKind.TYPE_PARAMETER) {
       return _searchReferences_Local(
           element, (n) => n.parent is CompilationUnit);
     }
     return const <SearchResult>[];
   }
 
+  /**
+   * Returns subtypes of the given [type].
+   */
+  Future<List<SearchResult>> subTypes(ClassElement type) async {
+    if (type == null) {
+      return const <SearchResult>[];
+    }
+    List<SearchResult> results = <SearchResult>[];
+    await _addResults(results, type, {
+      IndexRelationKind.IS_EXTENDED_BY: SearchResultKind.REFERENCE,
+      IndexRelationKind.IS_MIXED_IN_BY: SearchResultKind.REFERENCE,
+      IndexRelationKind.IS_IMPLEMENTED_BY: SearchResultKind.REFERENCE
+    });
+    return results;
+  }
+
   Future<Null> _addResults(List<SearchResult> results, Element element,
       Map<IndexRelationKind, SearchResultKind> relationToResultKind) async {
     String path = element.source.fullName;
 
     // If the file with the element is not known, then the element is not used.
     if (!_driver.knownFiles.contains(path)) {
       return;
     }
 
     // Prepare the element name.
@@ skipping 597 matching lines @@
   }
 
   void _addResult(AstNode node, SearchResultKind kind) {
     bool isQualified = node.parent is Label;
     Element enclosingElement =
         _getEnclosingElement(enclosingUnitElement, node.offset);
     results.add(new SearchResult._(element, enclosingElement, kind, node.offset,
         node.length, true, isQualified));
   }
 }