Use classList#toggle instead of classList[whatever ? 'add' : 'remove'].

chrome/browser/resources/chromeos/login/screen_password_changed.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Password changed screen implementation.
  */
 
 cr.define('login', function() {
   /**
@@ skipping 153 matching lines @@
 
   /**
    * Show password changed screen.
    * @param {boolean} showError Whether to show the incorrect password error.
    */
   PasswordChangedScreen.show = function(showError) {
     // We'll get here after the successful online authentication.
     // It assumes session is about to start so hides login screen controls.
     Oobe.getInstance().headerHidden = false;
     var screen = $('password-changed');
-    screen.classList[showError ? 'add' : 'remove']('password-error');
+    screen.classList.toggle('password-error', showError);
     screen.classList.add('migrate');
     screen.classList.remove('resync');
     $('old-password').value = '';
     $('password-changed').disabled = false;
 
     Oobe.showScreen({id: SCREEN_PASSWORD_CHANGED});
     $('password-changed-ok-button').disabled = true;
   };
 
   return {
     PasswordChangedScreen: PasswordChangedScreen
   };
 });