Use package:stack_trace in unittest.

pkg/unittest/lib/html_enhanced_config.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.
 
 /**
  * A simple unit test library for running tests in a browser.
  *
  * Provides enhanced HTML output with collapsible group headers
  * and other at-a-glance information about the test results.
  */
@@ skipping 252 matching lines @@
 
     if (!test_.isComplete) {
        addRowElement('${test_.id}', 'NO STATUS', 'Test did not complete.');
        return;
     }
 
     addRowElement('${test_.id}', '${test_.result.toUpperCase()}',
         '${test_.description}. ${_htmlEscape(test_.message)}');
 
     if (test_.stackTrace != null) {
-      addRowElement('', '', '<pre>${_htmlEscape(test_.stackTrace)}</pre>');
+      addRowElement('', '',
+          '<pre>${_htmlEscape(test_.stackTrace.toString())}</pre>');
     }
   }
 
 
   static bool get _isIE => window.navigator.userAgent.contains('MSIE');
 
   String get _htmlTestCSS =>
   '''
   body{
     font-size: 14px;
@@ skipping 132 matching lines @@
 
   ''';
 }
 
 void useHtmlEnhancedConfiguration([bool isLayoutTest = false]) {
   unittestConfiguration = isLayoutTest ? _singletonLayout : _singletonNotLayout;
 }
 
 final _singletonLayout = new HtmlEnhancedConfiguration(true);
 final _singletonNotLayout = new HtmlEnhancedConfiguration(false);