Restore missing properties

runtime/observatory/lib/src/models/objects/instance.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;
 
 enum InstanceKind {
   /// A general instance of the Dart class Object.
   plainInstance,
   /// null instance.
@@ skipping 290 matching lines @@
   /// [optional] The key for a WeakProperty instance.
   ///
   /// Provided for instance kinds:
   ///   WeakProperty
   InstanceRef get value;
 
   /// [optional] The referent of a MirrorReference instance.
   ///
   /// Provided for instance kinds:
   ///   MirrorReference
-  InstanceRef get referent;
+  ObjectRef get referent;
 }