Prepare for decoupling analyzer ASTs from element model.

pkg/analyzer/lib/src/dart/analysis/index.dart

Index: pkg/analyzer/lib/src/dart/analysis/index.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/index.dart b/pkg/analyzer/lib/src/dart/analysis/index.dart
index 8c8c132aefac169fdf9d03821f3c787c5e995dbe..04d33590e43284d4e810c0c09baf8162e1326e61 100644
--- a/pkg/analyzer/lib/src/dart/analysis/index.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/index.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/standard_resolution_map.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ -505,7 +506,10 @@ class _IndexContributor extends GeneralizingAstVisitor {
   @override
   visitClassDeclaration(ClassDeclaration node) {
     if (node.extendsClause == null) {
-      ClassElement objectElement = node.element.supertype?.element;
+      ClassElement objectElement = resolutionMap
+          .elementDeclaredByClassDeclaration(node)
+          .supertype
+          ?.element;
       recordRelationOffset(objectElement, IndexRelationKind.IS_EXTENDED_BY,
           node.name.offset, 0, true);
     }