reverted:
--- b/core/lib/Drupal/Core/Url.php
+++ a/core/lib/Drupal/Core/Url.php
@@ -277,10 +277,6 @@
     if (strpos($uri, '//') === 0) {
       $uri_parts['scheme'] = '';
     }
-    // We support root-relative URLs.
-    elseif (strpos($uri, '/') === 0) {
-      $uri_parts['scheme'] = 'base';
-    }
     elseif (empty($uri_parts['scheme'])) {
       throw new \InvalidArgumentException("The URI '$uri' is invalid. You must use a valid URI scheme.");
     }