diff --git a/core/includes/file.inc b/core/includes/file.inc
index d365ce8..10542db 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -216,23 +216,12 @@ function file_create_url($uri) {
  *   original value of $file_url.
  *
  * @see file_create_url()
+ *
+ * @deprecated in Drupal 8.0.x-dev and will be removed before Drupal 9.0.0.
+ *   Use\Drupal\Core\File\FileUrlGenerator::transformRelative().
  */
 function file_url_transform_relative($file_url) {
-  // Unfortunately, we pretty much have to duplicate Symfony's
-  // Request::getHttpHost() method because Request::getPort() may return NULL
-  // instead of a port number.
-  $request = \Drupal::request();
-  $host = $request->getHost();
-  $scheme = $request->getScheme();
-  $port = $request->getPort() ?: 80;
-  if (('http' == $scheme && $port == 80) || ('https' == $scheme && $port == 443)) {
-    $http_host = $host;
-  }
-  else {
-    $http_host = $host . ':' . $port;
-  }
-
-  return preg_replace('|^https?://' . $http_host . '|', '', $file_url);
+  return \Drupal::service('file_url_generator')->transformRelative($file_url);
 }
 
 /**
diff --git a/core/lib/Drupal/Core/File/FileUrlGenerator.php b/core/lib/Drupal/Core/File/FileUrlGenerator.php
index fefd33d..68562d1 100644
--- a/core/lib/Drupal/Core/File/FileUrlGenerator.php
+++ b/core/lib/Drupal/Core/File/FileUrlGenerator.php
@@ -81,6 +81,10 @@ public function __construct(FileSystemInterface $file_system, StreamWrapperManag
    *   The URI to a file for which we need an external URL, or the path to a
    *   shipped file.
    *
+   * @param bool $relative
+   *   Set to TRUE to return a URL that is relative to the current request's
+   *   base URI.
+   *
    * @return string
    *   A string containing a URL that may be used to access the file.
    *   If the provided string already contains a preceding 'http', 'https', or
@@ -90,7 +94,7 @@ public function __construct(FileSystemInterface $file_system, StreamWrapperManag
    * @see https://www.drupal.org/node/515192
    * @see file_url_transform_relative()
    */
-  public function generate($uri) {
+  public function generate($uri, $relative = FALSE) {
     // Allow the URI to be altered, e.g. to serve a file from a CDN or static
     // file server.
     $this->moduleHandler->alter('file_url', $uri);
@@ -130,6 +134,9 @@ public function generate($uri) {
     elseif ($scheme == 'http' || $scheme == 'https' || $scheme == 'data') {
       // Check for HTTP and data URI-encoded URLs so that we don't have to
       // implement getExternalUrl() for the HTTP and data schemes.
+      if ($relative) {
+        return $this->transformRelative($uri);
+      }
       return $uri;
     }
     else {
@@ -143,4 +150,36 @@ public function generate($uri) {
     }
   }
 
+  /**
+   * Transforms an absolute URL of a local file to a relative URL.
+   *
+   * May be useful to prevent problems on multisite set-ups and prevent mixed
+   * content errors when using HTTPS + HTTP.
+   *
+   * @param string $file_url
+   *   A file URL of a local file as generated by file_create_url().
+   *
+   * @return string
+   *   If the file URL indeed pointed to a local file and was indeed absolute,
+   *   then the transformed, relative URL to the local file. Otherwise: the
+   *   original value of $file_url.
+  */
+  public function transformRelative($file_url) {
+    // Unfortunately, we pretty much have to duplicate Symfony's
+    // Request::getHttpHost() method because Request::getPort() may return NULL
+    // instead of a port number.
+    $request = $this->requestStack->getCurrentRequest();
+    $host = $request->getHost();
+    $scheme = $request->getScheme();
+    $port = $request->getPort() ?: 80;
+    if (('http' == $scheme && $port == 80) || ('https' == $scheme && $port == 443)) {
+      $http_host = $host;
+    }
+    else {
+      $http_host = $host . ':' . $port;
+    }
+
+    return preg_replace('|^https?://' . $http_host . '|', '', $file_url);
+  }
+
 }