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

runtime/bin/vmservice/observatory/lib/src/elements/observatory_element.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 observatory_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:observatory/app.dart';
 import 'package:polymer/polymer.dart';
@@ skipping 87 matching lines @@
   String formatTimePrecise(double time) => Utils.formatTimePrecise(time);
 
   String formatTime(double time) => Utils.formatTime(time);
 
   String formatSeconds(double x) => Utils.formatSeconds(x);
 
 
   String formatSize(int bytes) => Utils.formatSize(bytes);
 
   String fileAndLine(Map frame) {
-    var file = frame['script']['user_name'];
+    var file = frame['script'].name;
     var shortFile = file.substring(file.lastIndexOf('/') + 1);
     return "${shortFile}:${frame['line']}";
   }
 
   int parseInt(String value) => int.parse(value);
 
   bool isNull(ref) {
-    return ref != null && ref.serviceType == 'Null' && ref.id == 'objects/null';
+    return ref != null && ref.serviceType == 'Null';
   }
 
-  bool isPsuedoNull(ref) {
-    return ref != null && ref.serviceType == 'Null' && ref.id != 'objects/null';
+  bool isSentinel(ref) {
+    return ref != null && ref.serviceType == 'Sentinel';
   }
 
   bool isError(ref) {
     return ref != null && ref.serviceType == 'Error';
   }
 
   bool isInt(ref) {
     return ref != null && (ref.serviceType == 'Smi' ||
                            ref.serviceType == 'Mint' ||
                            ref.serviceType == 'Bigint');
@@ skipping 20 matching lines @@
             ref.serviceType == 'Array');
   }
 
   bool isType(ref) {
     return ref != null && (ref.serviceType == 'Type');
   }
 
   bool isUnexpected(ref) {
     if (ref == null) return false;
     return (!['Null',
+              'Sentinel',
               'Smi',
               'Mint',
               'Bigint',
               'Bool',
               'String',
               'Double',
               'Instance',
               'GrowableObjectArray',
               'Array',
               'Type',
               'Error'].contains(ref.serviceType));
   }
 }