diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index 240aea9..d6ac7bd 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -822,14 +822,13 @@ protected function assertThemeOutput($callback, array $variables = array(), $exp
   }
 
   /**
-   * Asserts that a field exists in the current page by the given XPath.
+   * Asserts that a field exists in the current page with a given xpath result.
    *
-   * @param string $xpath
-   *   XPath used to find the field.
+   * @param \SimpleXmlElement[] $fields
+   *   Xml elements.
    * @param string $value
-   *   (optional) Value of the field to assert. You may pass in NULL (default)
-   *   to skip checking the actual value, while still checking that the field
-   *   exists.
+   *   Value of the field to assert. You may pass in NULL (default) to skip
+   *   checking the actual value, while still checking that the field exists.
    * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use format_string() to embed variables in the message text, not
@@ -843,9 +842,7 @@ protected function assertThemeOutput($callback, array $variables = array(), $exp
    * @return bool
    *   TRUE on pass, FALSE on fail.
    */
-  protected function assertFieldByXPath($xpath, $value = NULL, $message = '', $group = 'Other') {
-    $fields = $this->xpath($xpath);
-
+  protected function assertFieldsByValue($fields, $value = NULL, $message = '', $group = 'Other') {
     // If value specified then check array for match.
     $found = TRUE;
     if (isset($value)) {
@@ -881,6 +878,34 @@ protected function assertFieldByXPath($xpath, $value = NULL, $message = '', $gro
   }
 
   /**
+   * Asserts that a field exists in the current page by the given XPath.
+   *
+   * @param string $xpath
+   *   XPath used to find the field.
+   * @param string $value
+   *   (optional) Value of the field to assert. You may pass in NULL (default)
+   *   to skip checking the actual value, while still checking that the field
+   *   exists.
+   * @param string $message
+   *   (optional) A message to display with the assertion. Do not translate
+   *   messages: use format_string() to embed variables in the message text, not
+   *   t(). If left blank, a default message will be displayed.
+   * @param string $group
+   *   (optional) The group this message is in, which is displayed in a column
+   *   in test output. Use 'Debug' to indicate this is debugging output. Do not
+   *   translate this string. Defaults to 'Other'; most tests do not override
+   *   this default.
+   *
+   * @return bool
+   *   TRUE on pass, FALSE on fail.
+   */
+  protected function assertFieldByXPath($xpath, $value = NULL, $message = '', $group = 'Other') {
+    $fields = $this->xpath($xpath);
+
+    return $this->assertFieldsByValue($fields, $value, $message, $group);
+  }
+
+  /**
    * Get the selected value from a select field.
    *
    * @param \SimpleXmlElement $element
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 79f2e39..6578078 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -19,6 +19,7 @@
 use Drupal\Core\Database\Database;
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\EventSubscriber\AjaxSubscriber;
 use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Session\AccountInterface;
@@ -1680,7 +1681,7 @@ protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_p
         $extra_post[$key] = $value;
       }
     }
-    $extra_post['ajax'] = 1;
+    $extra_post[AjaxSubscriber::AJAX_REQUEST_PARAMETER] = 1;
     $extra_post += $this->getAjaxPageStatePostData();
     // Now serialize all the $extra_post values, and prepend it with an '&'.
     $extra_post = '&' . $this->serializePostValues($extra_post);
diff --git a/core/modules/system/src/Tests/Ajax/MultiFormTest.php b/core/modules/system/src/Tests/Ajax/MultiFormTest.php
index 449861c..87a00fe 100644
--- a/core/modules/system/src/Tests/Ajax/MultiFormTest.php
+++ b/core/modules/system/src/Tests/Ajax/MultiFormTest.php
@@ -58,10 +58,9 @@ function testMultiForm() {
     // each Ajax submission, but these variables are stable and help target the
     // desired elements.
     $field_name = 'field_ajax_test';
-    $field_xpaths = array(
-      'node-page-form' => '//form[@id="node-page-form"]//div[contains(@class, "field-name-field-ajax-test")]',
-      'node-page-form--2' => '//form[@id="node-page-form--2"]//div[contains(@class, "field-name-field-ajax-test")]',
-    );
+
+    $form_xpath = '//form[starts-with(@id, "node-page-form")]';
+    $field_xpath = '//div[contains(@class, "field-name-field-ajax-test")]';
     $button_name = $field_name . '_add_more';
     $button_value = t('Add another item');
     $button_xpath_suffix = '//input[@name="' . $button_name . '"]';
@@ -71,19 +70,28 @@ function testMultiForm() {
     // of field items and "add more" button for the multi-valued field within
     // each form.
     $this->drupalGet('form-test/two-instances-of-same-form');
-    foreach ($field_xpaths as $field_xpath) {
-      $this->assert(count($this->xpath($field_xpath . $field_items_xpath_suffix)) == 1, 'Found the correct number of field items on the initial page.');
-      $this->assertFieldByXPath($field_xpath . $button_xpath_suffix, NULL, 'Found the "add more" button on the initial page.');
+
+    $fields = $this->xpath($form_xpath . $field_xpath);
+    $this->assertEqual(count($fields), 2);
+    foreach ($fields as $field) {
+      $this->assertEqual(count($field->xpath('.' . $field_items_xpath_suffix)), 1, 'Found the correct number of field items on the initial page.');
+      $this->assertFieldsByValue($field->xpath('.' . $button_xpath_suffix), NULL, 'Found the "add more" button on the initial page.');
     }
+
     $this->assertNoDuplicateIds(t('Initial page contains unique IDs'), 'Other');
 
     // Submit the "add more" button of each form twice. After each corresponding
     // page update, ensure the same as above.
-    foreach ($field_xpaths as $form_html_id => $field_xpath) {
-      for ($i = 0; $i < 2; $i++) {
+    for ($i = 0; $i < 2; $i++) {
+      $forms = $this->xpath($form_xpath);
+      foreach ($forms as $offset => $form) {
+        $form_html_id = (string) $form['id'];
         $this->drupalPostAjaxForm(NULL, array(), array($button_name => $button_value), 'system/ajax', array(), array(), $form_html_id);
-        $this->assert(count($this->xpath($field_xpath . $field_items_xpath_suffix)) == $i+2, 'Found the correct number of field items after an AJAX submission.');
-        $this->assertFieldByXPath($field_xpath . $button_xpath_suffix, NULL, 'Found the "add more" button after an AJAX submission.');
+        $form = $this->xpath($form_xpath)[$offset];
+        $field = $form->xpath('.' . $field_xpath);
+
+        $this->assertEqual(count($field[0]->xpath('.' . $field_items_xpath_suffix)), $i+2, 'Found the correct number of field items after an AJAX submission.');
+        $this->assertFieldsByValue($field[0]->xpath('.' . $button_xpath_suffix), NULL, 'Found the "add more" button after an AJAX submission.');
         $this->assertNoDuplicateIds(t('Updated page contains unique IDs'), 'Other');
       }
     }