Begin implementing a debugger page in Observatory.

runtime/bin/vmservice/observatory/lib/src/service/object.dart

 // Copyright (c) 2014, 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 service;
 
 /// A [ServiceObject] is an object known to the VM service and is tied
 /// to an owning [Isolate].
 abstract class ServiceObject extends Observable {
   static int LexicalSortName(ServiceObject o1, ServiceObject o2) {
@@ skipping 212 matching lines @@
       return new Future.value(this);
     }
     if (_inProgressReload == null) {
       _inProgressReload = vm.getAsMap(link).then((ObservableMap map) {
           var mapType = _stripRef(map['type']);
           if (mapType != _type) {
             // If the type changes, return a new object instead of
             // updating the existing one.
             //
             // TODO(turnidge): Check for vmType changing as well?
-            assert(mapType == 'Error' || mapType == 'null');
+            assert(mapType == 'Error' || mapType == 'Sentinel');
             return new ServiceObject._fromMap(owner, map);
           }
           update(map);
           return this;
       }).whenComplete(() {
           // This reload is complete.
           _inProgressReload = null;
       });
     }
     return _inProgressReload;
@@ skipping 2481 matching lines @@
     var v = list[i];
     if ((v is ObservableMap) && _isServiceMap(v)) {
       list[i] = owner.getFromMap(v);
     } else if (v is ObservableList) {
       _upgradeObservableList(v, owner);
     } else if (v is ObservableMap) {
       _upgradeObservableMap(v, owner);
     }
   }
 }