diff --git a/core/modules/simpletest/src/TestFileCreationTrait.php b/core/modules/simpletest/src/TestFileCreationTrait.php
index 932a26d..e552b90 100644
--- a/core/modules/simpletest/src/TestFileCreationTrait.php
+++ b/core/modules/simpletest/src/TestFileCreationTrait.php
@@ -15,7 +15,7 @@
    * Gets a list of files that can be used in tests.
    *
    * The first time this method is called, it will call
-   * simpletest_generate_file() to generate binary and ASCII text files in the
+   * $this->generateFile() to generate binary and ASCII text files in the
    * public:// directory. It will also copy all files in
    * core/modules/simpletest/files to public://. These contain image, SQL, PHP,
    * JavaScript, and HTML files.
@@ -50,14 +50,14 @@ protected function drupalGetTestFiles($type, $size = NULL) {
       $lines = array(64, 1024);
       $count = 0;
       foreach ($lines as $line) {
-        simpletest_generate_file('binary-' . $count++, 64, $line, 'binary');
+        $this->generateFile('binary-' . $count++, 64, $line, 'binary');
       }
 
       // Generate ASCII text test files.
       $lines = array(16, 256, 1024, 2048, 20480);
       $count = 0;
       foreach ($lines as $line) {
-        simpletest_generate_file('text-' . $count++, 64, $line, 'text');
+        $this->generateFile('text-' . $count++, 64, $line, 'text');
       }
 
       // Copy other test files from simpletest.
@@ -103,4 +103,52 @@ protected function drupalCompareFiles($file1, $file2) {
       return strnatcmp($file1->name, $file2->name);
     }
   }
+
+  /**
+   * Generates a test file.
+   *
+   * @param string $filename
+   *   The name of the file, including the path. The suffix '.txt' is appended
+   *   to the supplied file name and the file is put into the public:// files
+   *   directory.
+   * @param int $width
+   *   The number of characters on one line.
+   * @param int $lines
+   *   The number of lines in the file.
+   * @param string $type
+   *   (optional) The type, one of:
+   *   - text: The generated file contains random ASCII characters.
+   *   - binary: The generated file contains random characters whose codes are
+   *     in the range of 0 to 31.
+   *   - binary-text: The generated file contains random sequence of '0' and '1'
+   *     values.
+   *
+   * @return string
+   *   The name of the file, including the path.
+   */
+  public static function generateFile($filename, $width, $lines, $type = 'binary-text') {
+    $text = '';
+    for ($i = 0; $i < $lines; $i++) {
+      // Generate $width - 1 characters to leave space for the "\n" character.
+      for ($j = 0; $j < $width - 1; $j++) {
+        switch ($type) {
+          case 'text':
+            $text .= chr(rand(32, 126));
+            break;
+          case 'binary':
+            $text .= chr(rand(0, 31));
+            break;
+          case 'binary-text':
+          default:
+            $text .= rand(0, 1);
+            break;
+        }
+      }
+      $text .= "\n";
+    }
+
+    // Create filename.
+    file_put_contents('public://' . $filename . '.txt', $text);
+    return $filename;
+  }
 }