Update to Drupal 8.1.5. For more information, see https://www.drupal.org/project/drupal/releases/8.1.5

core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\FunctionalJavascriptTests;
+
+use Drupal\Tests\WebAssert;
+
+/**
+ * Defines a class with methods for asserting presence of elements during tests.
+ */
+class JSWebAssert extends WebAssert {
+
+  /**
+   * Waits for AJAX request to be completed.
+   *
+   * @param int $timeout
+   *   (Optional) Timeout in milliseconds, defaults to 10000.
+   * @param string $message
+   *   (optional) A message for exception.
+   *
+   * @throws \RuntimeException
+   *   When the request is not completed. If left blank, a default message will
+   *   be displayed.
+   */
+  public function assertWaitOnAjaxRequest($timeout = 10000, $message = 'Unable to complete AJAX request.') {
+    $result = $this->session->wait($timeout, '(typeof(jQuery)=="undefined" || (0 === jQuery.active && 0 === jQuery(\':animated\').length))');
+    if (!$result) {
+      throw new \RuntimeException($message);
+    }
+  }
+
+}

core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php

@@ -33,6 +33,24 @@ abstract class JavascriptTestBase extends BrowserTestBase {
     return parent::initMink();
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function tearDown() {
+    // Wait for all requests to finish. It is possible that an AJAX request is
+    // still on-going.
+    $result = $this->getSession()->wait(5000, '(typeof(jQuery)=="undefined" || (0 === jQuery.active && 0 === jQuery(\':animated\').length))');
+    if (!$result) {
+      // If the wait is unsuccessful, there may still be an AJAX request in
+      // progress. If we tear down now, then this AJAX request may fail with
+      // missing database tables, because tear down will have removed them. Rather
+      // than allow it to fail, throw an explicit exception now explaining what
+      // the problem is.
+      throw new \RuntimeException('Unfinished AJAX requests whilst tearing down a test');
+    }
+    parent::tearDown();
+  }
+
   /**
    * Asserts that the element with the given CSS selector is visible.
    *
@@ -84,4 +102,11 @@ abstract class JavascriptTestBase extends BrowserTestBase {
     $this->assertTrue($result, $message);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function assertSession($name = NULL) {
+    return new JSWebAssert($this->getSession($name), $this->baseUrl);
+  }
+
 }