Supporting both FieldRef and InstanceRef in RetainingPathItem.parentField

runtime/observatory/tests/observatory_ui/mocks/objects/retaining_path.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.
 
 part of mocks;
 
 class RetainingPathMock implements M.RetainingPath {
   final Iterable<M.RetainingPathItem> elements;
 
   const RetainingPathMock({this.elements: const []});
 }
 
 class RetainingPathItemMock implements M.RetainingPathItem {
   final M.ObjectRef source;
-  final M.InstanceRef parentField;
+  final M.ObjectRef parentField;
   final int parentListIndex;
   final int parentWordOffset;
 
   const RetainingPathItemMock({this.source: const InstanceRefMock(),
                                this.parentField, this.parentListIndex,
                                this.parentWordOffset});
 }