diff --git a/core/modules/node/lib/Drupal/node/Tests/NodeTypeRenameConfigImportTest.php b/core/modules/node/lib/Drupal/node/Tests/NodeTypeRenameConfigImportTest.php
index 2bb230e..072e8f3 100644
--- a/core/modules/node/lib/Drupal/node/Tests/NodeTypeRenameConfigImportTest.php
+++ b/core/modules/node/lib/Drupal/node/Tests/NodeTypeRenameConfigImportTest.php
@@ -9,7 +9,6 @@
 
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Unicode;
-use Drupal\Component\Utility\Xss;
 use Drupal\Core\Config\Entity\ConfigEntityStorage;
 use Drupal\simpletest\WebTestBase;
 
@@ -126,27 +125,4 @@ public function testConfigurationRename() {
     $this->assertIdentical($staged_type, $content_type->type);
   }
 
-  /**
-   * Asserts that a Perl regex pattern is found in the text content.
-   *
-   * @param string $pattern
-   *   Perl regex to look for including the regex delimiters.
-   * @param string $message
-   *   (optional) A message to display with the assertion.
-   *
-   * @return bool
-   *   TRUE on pass, FALSE on failure.
-   */
-  protected function assertTextPattern($pattern, $message = NULL) {
-    // @see WebTestBase::assertTextHelper()
-    if ($this->plainTextContent === FALSE) {
-      $this->plainTextContent = Xss::filter($this->drupalGetContent(), array());
-    }
-    // @see WebTestBase::assertPattern()
-    if (!$message) {
-      $message = String::format('Pattern "@pattern" found', array('@pattern' => $pattern));
-    }
-    return $this->assert((bool) preg_match($pattern, $this->plainTextContent), $message);
-  }
-
 }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/AssertContentTrait.php b/core/modules/simpletest/lib/Drupal/simpletest/AssertContentTrait.php
index 47b3f95..1249f95 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/AssertContentTrait.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/AssertContentTrait.php
@@ -80,6 +80,22 @@ protected function getTextContent() {
   }
 
   /**
+   * Removes all white-space between HTML tags from the raw content.
+   *
+   * White-space is only removed if there are no non-white-space characters
+   * between HTML tags.
+   *
+   * Use this (once) after performing an operation that sets new raw content,
+   * and when you want to use e.g. assertText() but ignore potential white-space
+   * caused by HTML output templates.
+   */
+  protected function removeWhiteSpace() {
+    $this->content = preg_replace('@>\s+<@', '><', $this->content);
+    $this->plainTextContent = NULL;
+    $this->elements = NULL;
+  }
+
+  /**
    * Gets the value of drupalSettings for the currently-loaded page.
    *
    * drupalSettings refers to the drupalSettings JavaScript variable.
@@ -606,6 +622,29 @@ protected function assertNoPattern($pattern, $message = '', $group = 'Other') {
   }
 
   /**
+   * Asserts that a Perl regex pattern is found in the plain-text content.
+   *
+   * @param string $pattern
+   *   Perl regex to look for including the regex delimiters.
+   * @param string $message
+   *   (optional) A message to display with the assertion.
+   * @param $group
+   *   (optional) The group this message is in, which is displayed in a column
+   *   in test output. Use 'Debug' to indicate this is debugging output. Do not
+   *   translate this string. Defaults to 'Other'; most tests do not override
+   *   this default.
+   *
+   * @return bool
+   *   TRUE on pass, FALSE on failure.
+   */
+  protected function assertTextPattern($pattern, $message = NULL, $group = 'Other') {
+    if (!isset($message)) {
+      $message = String::format('Pattern "@pattern" found', array('@pattern' => $pattern));
+    }
+    return $this->assert((bool) preg_match($pattern, $this->getTextContent()), $message, $group);
+  }
+
+  /**
    * Pass if the page title is the given string.
    *
    * @param $title
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php
index e7d62f3..36d4f21 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php
@@ -137,11 +137,4 @@ function testThemeTableHeaderCellOption() {
     $this->assertRaw('<th>1</th><td>1</td><td>1</td>', 'The th and td tags was printed correctly.');
   }
 
-  /**
-   * Removes all white-space between HTML tags from $this->content.
-   */
-  protected function removeWhiteSpace() {
-    $this->content = preg_replace('@>\s+<@', '><', $this->content);
-  }
-
 }