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

chrome/browser/resources/chromeos/login/bubble.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 Bubble implementation.
  */
 
 // TODO(xiyuan): Move this into shared.
 cr.define('cr.ui', function() {
@@ skipping 81 matching lines @@
       e.stopPropagation();
     },
 
     /**
      * Sets the attachment of the bubble.
      * @param {!Attachment} attachment Bubble attachment.
      */
     setAttachment_: function(attachment) {
       for (var k in Bubble.Attachment) {
         var v = Bubble.Attachment[k];
-        this.classList[v == attachment ? 'add' : 'remove'](v);
+        this.classList.toggle(v, v == attachment);
       }
     },
 
     /**
      * Shows the bubble for given anchor element.
      * @param {!Object} pos Bubble position (left, top, right, bottom in px).
      * @param {!Attachment} attachment Bubble attachment (on which side of the
      *     specified position it should be displayed).
      * @param {HTMLElement} opt_content Content to show in bubble.
      *     If not specified, bubble element content is shown.
@@ skipping 191 matching lines @@
     handleWindowBlur_: function(e) {
       if (!this.hidden)
         this.hide();
     }
   };
 
   return {
     Bubble: Bubble
   };
 });