Use package:stack_trace in unittest.

pkg/unittest/lib/html_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.
  */
 library unittest_html_config;
 
 import 'dart:async';
@@ skipping 62 matching lines @@
   }
 
   var html = '''
       <tr>
         <td>${test_.id}</td>
         <td class="unittest-${test_.result}">${test_.result.toUpperCase()}</td>
         <td>Expectation: <a href="#testFilter=${test_.description}">${test_.description}</a>. ${_htmlEscape(test_.message)}</td>
       </tr>''';
 
   if (test_.stackTrace != null) {
-    html = '$html<tr><td></td><td colspan="2"><pre>${_htmlEscape(test_.stackTrace)}</pre></td></tr>';
+    html = '$html<tr><td></td><td colspan="2"><pre>' +
+        _htmlEscape(test_.stackTrace.toString()) +
+        '</pre></td></tr>';
   }
 
   return html;
 }
 
 //TODO(pquitslund): Move to a common lib
 String _htmlEscape(String string) {
   return string.replaceAll('&', '&amp;')
                .replaceAll('<','&lt;')
                .replaceAll('>','&gt;');
@@ skipping 68 matching lines @@
     window.postMessage('unittest-suite-done', '*');
   }
 }
 
 void useHtmlConfiguration([bool isLayoutTest = false]) {
   unittestConfiguration = isLayoutTest ? _singletonLayout : _singletonNotLayout;
 }
 
 final _singletonLayout = new HtmlConfiguration(true);
 final _singletonNotLayout = new HtmlConfiguration(false);