Fix CrSettingsLanguagesPageTest dialog flake

chrome/test/data/webui/settings/languages_page_tests.js

 // Copyright 2017 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.
 
 cr.define('languages_page_tests', function() {
   /** @enum {string} */
   const TestNames = {
     AddLanguagesDialog: 'add languages dialog',
     LanguageMenu: 'language menu',
     InputMethods: 'input methods',
@@ skipping 90 matching lines @@
 
           // No languages have been checked, so the action button is disabled.
           assertTrue(actionButton.disabled);
           assertFalse(cancelButton.disabled);
 
           done();
         });
       });
 
       // After every test, check that the dialog is removed from the DOM.
-      teardown(function() {
+      teardown(function(done) {
+        // Flush the DOM, then wait for the |restamp| to take effect.
         Polymer.dom.flush();
-        assertEquals(null, languagesPage.$$('settings-add-languages-dialog'));
+        setTimeout(function() {

[dpapad, 2017/05/09 01:14:29]

Nit (optional): Perhaps it would be more robust to listen for a dom-change event
on the dom-if element itself.

[michaelpg, 2017/05/16 00:03:44]

The MutationObserver you suggested in the bug works on the language page's
shadow root - LGTY?

+          assertEquals(null, languagesPage.$$('settings-add-languages-dialog'));
+          done();
+        });
       });
 
       test('cancel', function() {
         // Canceling the dialog should close and remove it.
         MockInteractions.tap(cancelButton);
       });
 
       test('add languages and cancel', function(done) {
         // Check some languages.
         MockInteractions.tap(dialogItems[1]);  // en-CA.
@@ skipping 262 matching lines @@
         // Now the secondary row is empty, so it shouldn't be shown.
         assertFalse(triggerRow.classList.contains('two-line'));
         assertEquals(
             0, triggerRow.querySelector('.secondary').textContent.length);
       }
     });
   });
 
   return {TestNames: TestNames};
 });