diff --git a/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
index c6a5f05..7add013 100644
--- a/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -298,6 +298,10 @@ public static function fromUri($uri, $options = []) {
     if ($uri_parts === FALSE) {
       throw new \InvalidArgumentException(SafeMarkup::format('The URI "@uri" is malformed.', ['@uri' => $uri]));
     }
+    // Support for protocol-relative URLs.
+    if (empty($uri_parts['scheme']) &&  strpos($uri, '//') === 0) {
+      $uri_parts['scheme'] = 'relative';
+    }
     if (empty($uri_parts['scheme'])) {
       throw new \InvalidArgumentException(SafeMarkup::format('The URI "@uri" is invalid. You must use a valid URI scheme.', ['@uri' => $uri]));
     }