Supporting both FieldRef and InstanceRef in RetainingPathItem.parentField

runtime/observatory/lib/src/models/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 models;
 
 abstract class RetainingPath {
   Iterable<RetainingPathItem> get elements;
 }
 
 abstract class RetainingPathItem {
   ObjectRef get source;
   /// [optional]
-  InstanceRef get parentField;
+  /// FieldRef or InstanceRef (null)
+  ObjectRef get parentField;
   /// [optional]
   int get parentListIndex;
   /// [optional]
   int get parentWordOffset;
 }