Display isolate message queue in Observatory debugger

runtime/observatory/lib/src/elements/script_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 script_ref_element;
 
 import 'package:polymer/polymer.dart';
 import 'package:observatory/service.dart';
 import 'service_ref.dart';
 
 @CustomTag('script-ref')
 class ScriptRefElement extends ServiceRefElement {
-  @published int pos = -1;
+  @published int pos;
 
   String get hoverText {
     if (ref == null) {
       return super.hoverText;
     }
     return ref.vmName;
   }
 
   void posChanged(oldValue) {
     _updateProperties(null);
   }
 
   void _updateProperties(_) {
     if (ref != null && ref.loaded) {
       notifyPropertyChange(#name, 0, 1);
       notifyPropertyChange(#url, 0, 1);
     }
   }
 
   String get name {
     if (ref == null) {
       return super.name;
     }
-    if (pos >= 0) {
+    if ((pos != null) && (pos >= 0)) {
       if (ref.loaded) {
         // Script is loaded, get the line number.
         Script script = ref;
         return '${super.name}:${script.tokenToLine(pos)}:'
             '${script.tokenToCol(pos)}';
       } else {
         ref.load().then(_updateProperties);
       }
     }
     return super.name;
   }
 
   String get url {
     if (ref == null) {
       return super.url;
     }
-    if (pos >= 0) {
+    if ((pos != null) && (pos >= 0)) {
       if (ref.loaded) {
         return '${super.url}---pos=${pos}';
       } else {
         ref.load().then(_updateProperties);
       }
     }
     return super.url;
   }
 
   ScriptRefElement.created() : super.created();
 }