Prepare for decoupling analyzer ASTs from element model.

pkg/analysis_server/lib/src/services/index/index_unit.dart

Index: pkg/analysis_server/lib/src/services/index/index_unit.dart
diff --git a/pkg/analysis_server/lib/src/services/index/index_unit.dart b/pkg/analysis_server/lib/src/services/index/index_unit.dart
index 5a9cb479fa000cae4ad9a4bb814e3b2c85056eab..797e014bb624925097b7845308ae6500e060e1e6 100644
--- a/pkg/analysis_server/lib/src/services/index/index_unit.dart
+++ b/pkg/analysis_server/lib/src/services/index/index_unit.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/resolution_accessors.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ -489,7 +490,8 @@ class _IndexContributor extends _IndexDeclarationContributor {
   @override
   visitClassDeclaration(ClassDeclaration node) {
     if (node.extendsClause == null) {
-      ClassElement objectElement = node.element.supertype?.element;
+      ClassElement objectElement =
+          elementForClassDeclaration(node).supertype?.element;
       recordRelationOffset(objectElement, IndexRelationKind.IS_EXTENDED_BY,
           node.name.offset, 0, true);
     }