diff --git a/core/modules/book/tests/src/FunctionalJavascript/BookJavascriptTest.php b/core/modules/book/tests/src/FunctionalJavascript/BookJavascriptTest.php
index af32ea6..b324e72 100644
--- a/core/modules/book/tests/src/FunctionalJavascript/BookJavascriptTest.php
+++ b/core/modules/book/tests/src/FunctionalJavascript/BookJavascriptTest.php
@@ -59,7 +59,7 @@ public function testBookOrdering() {
     $this->assertGreaterThan($weight_select1->getValue(), $weight_select2->getValue());
 
     // Check that '1st page' precedes the '2nd page'.
-    $this->assertOrderInPage(['1st page', '2nd page']);
+    $this->assertSession()->orderInPage(['1st page', '2nd page']);
 
     // Check that the 'unsaved changes' text is not present in the message area.
     $this->assertSession()->pageTextNotContains('You have unsaved changes.');
@@ -79,7 +79,7 @@ public function testBookOrdering() {
     $this->assertSession()->pageTextContains('You have unsaved changes.');
 
     // Check that '2nd page' page precedes the '1st page'.
-    $this->assertOrderInPage(['2nd page', '1st page']);
+    $this->assertSession()->orderInPage(['2nd page', '1st page']);
 
     // Toggle row weight selects as visible.
     $page->findButton('Show row weights')->click();
@@ -102,7 +102,7 @@ public function testBookOrdering() {
     $this->assertSession()->pageTextContains(new FormattableMarkup('Updated book @book.', ['@book' => $book->getTitle()]));
 
     // Check again that '2nd page' is on top after form submit.
-    $this->assertOrderInPage(['2nd page', '1st page']);
+    $this->assertSession()->orderInPage(['2nd page', '1st page']);
 
     // Check that page reordering was done in the backend.
     $page1 = Node::load($page1->id());
@@ -110,30 +110,4 @@ public function testBookOrdering() {
     $this->assertGreaterThan($page2->book['weight'], $page1->book['weight']);
   }
 
-  /**
-   * Asserts that several pieces of markup are in a given order in the page.
-   *
-   * @param string[] $items
-   *   An ordered list of strings.
-   *
-   * @throws \Behat\Mink\Exception\ExpectationException
-   *   When any of the given string is not found.
-   */
-  protected function assertOrderInPage(array $items) {
-    $session = $this->getSession();
-    $text = $session->getPage()->getHtml();
-    $strings = [];
-    foreach ($items as $item) {
-      if (($pos = strpos($text, $item)) === FALSE) {
-        throw new ExpectationException("Cannot find '$item' in the page", $session->getDriver());
-      }
-      $strings[$pos] = $item;
-    }
-    ksort($strings);
-    $ordered = implode(', ', array_map(function ($item) {
-      return "'$item'";
-    }, $items));
-    $this->assertSame($items, array_values($strings), "Found strings, ordered as: $ordered.");
-  }
-
 }
diff --git a/core/tests/Drupal/Tests/WebAssert.php b/core/tests/Drupal/Tests/WebAssert.php
index 6156dda..f967e4d 100644
--- a/core/tests/Drupal/Tests/WebAssert.php
+++ b/core/tests/Drupal/Tests/WebAssert.php
@@ -422,4 +422,31 @@ public function fieldDisabled($field, TraversableElement $container = NULL)  {
     return $node;
   }
 
+  /**
+   * Asserts that several pieces of markup are in a given order in the page.
+   *
+   * @param string[] $items
+   *   An ordered list of strings.
+   *
+   * @throws \Behat\Mink\Exception\ExpectationException
+   *   When any of the given string is not found.
+   */
+  public function orderInPage(array $items) {
+    $text = $this->session->getPage()->getHtml();
+    $strings = [];
+    foreach ($items as $item) {
+      if (($pos = strpos($text, $item)) === FALSE) {
+        throw new ExpectationException("Cannot find '$item' in the page", $this->session->getDriver());
+      }
+      $strings[$pos] = $item;
+    }
+    ksort($strings);
+    $ordered = implode(', ', array_map(function ($item) {
+      return "'$item'";
+    }, $items));
+    if ($items !== array_values($strings)) {
+      throw new ExpectationException("Strings were not correctly ordered as: $ordered.", $this->session->getDriver());
+    }
+  }
+
 }