Fixed stack trace for isolates without any Dart frames.

runtime/bin/vmstats/vmstats.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 dart.vmstats;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:json' as JSON;
 
 part 'bargraph.dart';
 part 'isolate_list.dart';
 part 'models.dart';
 
 BarGraph _graph;
 IsolateList _isolateList;
 DivElement _statusText;
 IsolateListModel _isolates;
 Timer _updater;
 
-final int _POLL_INTERVAL = const Duration(seconds: 1);
+final int _POLL_INTERVAL = const Duration(seconds: 3);
 final String CYAN = '#00EE76';
 final String GREEN = '#97FFFF';
 
 void main() {
   DivElement dashBoard = query('#dashboard');
   CanvasElement canvas = query('#graph');
   var elements = [ new Element("Old Space", GREEN),
                    new Element("New Space", CYAN)];
   _graph = new BarGraph(canvas, elements);
   _isolateList = new IsolateList(query('#isolateList'));
@@ skipping 19 matching lines @@
 
 void onRequestFailed() {
   _updater.cancel();
   _isolates.removeListener(onUpdateStatus);
   showStatus('Server closed');
 }
 
 void showStatus(status) {
   _statusText.text = status;
 }