Add a new Sentinel type for sentinel responses. (They are not Null).

tests/standalone/vmservice/isolate_bad_object_test.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 isolate_bad_code_test;
 
 import 'dart:async';
 import 'test_helper.dart';
 import 'package:expect/expect.dart';
 
-class NullCollectionTest extends VmServiceRequestHelper {
-  NullCollectionTest(port, id) :
+class ExpiredCollectionTest extends VmServiceRequestHelper {
+  ExpiredCollectionTest(port, id) :
       super('http://127.0.0.1:$port/$id/objects/50');
 
   onRequestCompleted(Map reply) {
-    Expect.equals('Null', reply['type']);
+    Expect.equals('Sentinel', reply['type']);
+    Expect.equals('<expired>', reply['valueAsString']);
   }
 }
 
 class BadCollectionTest extends VmServiceRequestHelper {
   BadCollectionTest(port, id) :
       super('http://127.0.0.1:$port/$id/objects');
 
   onRequestCompleted(Map reply) {
     Expect.equals('Error', reply['type']);
   }
@@ skipping 10 matching lines @@
   }
 }
 
 main() {
   var process = new TestLauncher('unknown_isolate_command_script.dart');
   process.launch().then((port) {
     var test = new VMTest(port);
     test.makeRequest().then((_) {
       var badCollectionRequest =
           new BadCollectionTest(port, test._isolateId).makeRequest();
-      var nullCollectionRequest =
-          new NullCollectionTest(port, test._isolateId).makeRequest();
-      var requests = Future.wait([badCollectionRequest, nullCollectionRequest]);
+      var expiredCollectionRequest =
+          new ExpiredCollectionTest(port, test._isolateId).makeRequest();
+      var requests = Future.wait([badCollectionRequest,
+                                  expiredCollectionRequest]);
       requests.then((_) {
         process.requestExit();
       });
     });
   });
 }