diff --git a/core/lib/Drupal/Core/StreamWrapper/PublicStream.php b/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
index c18e880..ea7d1e6 100644
--- a/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
+++ b/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
@@ -59,17 +59,37 @@ public function getExternalUrl() {
   /**
    * Returns the base path for public://.
    *
+   * If we have a setting for the public:// scheme's path, we use that.
+   * Otherwise we build a reasonable default based on the site.path service if
+   * it's available, or a default behavior based on the request.
+   *
+   * The site path is injectable from the site.path service:
+   * @code
+   * $base_path = PublicStream::basePath(\Drupal::service('site.path'));
+   * @endcode
+   *
+   * @param \SplString $site_path
+   *   (optional) The site.path service parameter, which is typically the path
+   *   to sites/ in a Drupal installation. This allows you to inject the site
+   *   path using services from the caller. If omitted, this method will use the
+   *   global service container or the kernel's default behavior to determine
+   *   the site path.
+   *
    * @return string
    *   The base path for public:// typically sites/default/files.
    */
-  public static function basePath() {
-    // Find the site path. Kernel service is not always available at this point,
-    // but is preferred, when available.
-    if (\Drupal::hasService('kernel')) {
-      $site_path = \Drupal::service('site.path');
-    }
-    else {
-      $site_path = DrupalKernel::findSitePath(Request::createFromGlobals());
+  public static function basePath(\SplString $site_path = NULL) {
+    if ($site_path === NULL) {
+      // Find the site path. Kernel service is not always available at this
+      // point, but is preferred, when available.
+      if (\Drupal::hasService('kernel')) {
+        $site_path = \Drupal::service('site.path');
+      }
+      else {
+        // If there is no kernel available yet, we call the static
+        // findSitePath().
+        $site_path = DrupalKernel::findSitePath(Request::createFromGlobals());
+      }
     }
     return Settings::get('file_public_path', $site_path . '/files');
   }