Kim Pepper
--- a/core/lib/Drupal/Core/File/FileSystem.php
+++ b/core/lib/Drupal/Core/File/FileSystem.php
+  /**
+   * Determines if a directory is writable by the web server.
+   *
+   * PHP's is_writable() does not fully support stream wrappers, so this
+   * function fills that gap.
+   * In order to be able to write files within the directory, the directory
+   * itself must be writable, and it must also have the executable bit set. This
+   * helper function checks both at the same time.
+   *
+   * @param string $uri
+   *   A URI or pathname pointing to the directory that will be checked.
+   *
+   * @return bool
+   *   TRUE if the directory is writable and executable; FALSE otherwise.
+   */
+  public function isWritable($uri) {
--- a/core/lib/Drupal/Core/Updater/Updater.php
+++ b/core/lib/Drupal/Core/Updater/Updater.php
    // Make the parent dir writable if need be and create the dir.
    if (!is_dir($directory)) {
      $parent_dir = dirname($directory);
-      if (!is_writable($parent_dir)) {
+      if (!\Drupal::service('file_system')->isWritable($parent_dir)) {
--- a/core/modules/config/src/Form/ConfigImportForm.php
+++ b/core/modules/config/src/Form/ConfigImportForm.php
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $directory = $this->settings->get('config_sync_directory');
-    $directory_is_writable = is_writable($directory);
+    $directory_is_writable = \Drupal::service('file_system')->isWritable($directory);
--- a/core/modules/media/media.install
+++ b/core/modules/media/media.install
  if ($phase == 'install') {
    $destination = 'public://media-icons/generic';
    \Drupal::service('file_system')->prepareDirectory($destination, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS);
-    $is_writable = is_writable($destination);
+    $is_writable = \Drupal::service('file_system')->isWritable($destination);
--- a/core/modules/system/src/Form/PerformanceForm.php
+++ b/core/modules/system/src/Form/PerformanceForm.php
    ];

    $directory = 'assets://';
-    $is_writable = is_dir($directory) && is_writable($directory);
+    $is_writable = is_dir($directory) && \Drupal::service('file_system')->isWritable($directory);
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
    if ($phase == 'install') {
      \Drupal::service('file_system')->prepareDirectory($directory, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS);
    }
-    $is_writable = is_writable($directory);
+    $is_writable = \Drupal::service('file_system')->isWritable($directory);