Remove support for attribute event listeners.

client/testing/unittest/unittest.dart

 // Copyright (c) 2011, 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");
 
 #import("dart:dom");
@@ skipping 23 matching lines @@
       _nextBatch();
     }
   }
 }
 
 /** Runs all queued tests, one at a time. */
 _platformStartTests() {
   window.postMessage('unittest-suite-wait-for-done', '*');
 
   // Listen for uncaught errors.
-  window.onerror = _onErrorClosure;
+  window.addEventListener('error', _onErrorClosure, true);
 }
 
 _platformCompleteTests(int testsPassed, int testsFailed, int testsErrors) {
-  window.onerror = null;
+  window.removeEventListener('error', _onErrorClosure);
 
   if (_isLayoutTest && testsPassed == _tests.length) {
     document.body.innerHTML = "PASS";
   } else {
     var newBody = new StringBuffer();
     newBody.add("<table class='unittest-table'><tbody>");
     newBody.add(testsPassed == _tests.length
         ? "<tr><td colspan='3' class='unittest-pass'>PASS</td></tr>"
         : "<tr><td colspan='3' class='unittest-fail'>FAIL</td></tr>");
 
@@ skipping 36 matching lines @@
         <td>Expectation: ${test.description}. ${test.message}</td>
       </tr>''';
 
   if (test.stackTrace != null) {
     html +=
         '<tr><td></td><td colspan="2"><pre>${test.stackTrace}</pre></td></tr>';
   }
 
   return html;
 }