Allow observatory to distinguish between user-level and vm-level types.

runtime/bin/vmservice/observatory/lib/src/elements/instance_ref.dart

 // Copyright (c) 2013, 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.
 
 library instance_ref_element;
 
 import 'package:polymer/polymer.dart';
 import 'package:observatory/service.dart';
 import 'service_ref.dart';
 
 @CustomTag('instance-ref')
 class InstanceRefElement extends ServiceRefElement {
   InstanceRefElement.created() : super.created();
 
   String get hoverText {
     if (ref != null) {
-      if (ref.serviceType == 'Null') {
+      if (ref.type == 'Sentinel') {
         if (ref.id == 'objects/optimized-out') {
           return 'This object is no longer needed and has been removed by the optimizing compiler.';
         } else if (ref.id == 'objects/collected') {
           return 'This object has been reclaimed by the garbage collector.';
         } else if (ref.id == 'objects/expired') {
           return 'The handle to this object has expired.  Consider refreshing the page.';
         } else if (ref.id == 'objects/not-initialized') {
           return 'This object will be initialized once it is accessed by the program.';
         } else if (ref.id == 'objects/being-initialized') {
           return 'This object is currently being initialized.';
@@ skipping 20 matching lines @@
         notifyPropertyChange(#ref, 0, 1);
       }).whenComplete(done);
     } else {
       ServiceMap refMap = ref;
       refMap['fields'] = null;
       refMap['elements'] = null;
       done();
     }
   }
 }