reverted:
--- b/core/lib/Drupal/Component/Utility/Url.php
+++ a/core/lib/Drupal/Component/Utility/Url.php
@@ -206,11 +206,10 @@
    */
   public static function isExternal($path) {
     $colonpos = strpos($path, ':');
-    $scheme_relative = strpos($path, '//') === 0;
     // Avoid calling stripDangerousProtocols() if there is any
     // slash (/), hash (#) or question_mark (?) before the colon (:)
     // occurrence - if any - as this would clearly mean it is not a URL.
+    return $colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && static::stripDangerousProtocols($path) == $path;
-    return ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) || $scheme_relative) && static::stripDangerousProtocols($path) == $path;
   }
 
   /**
@@ -331,7 +330,7 @@
     if ($absolute) {
       return (bool) preg_match("
         /^                                                      # Start at the beginning of the text
+        (?:ftp|https?|feed):\/\/                                # Look for ftp, http, https or feed schemes
-        (?:ftp:|https?:|feed:)?\/\/                             # Look for ftp, http, https, feed or scheme relative schemes
         (?:                                                     # Userinfo (optional) which is typically
           (?:(?:[\w\.\-\+!$&'\(\)*\+,;=]|%[0-9a-f]{2})+:)*      # a username or a username and password
           (?:[\w\.\-\+%!$&'\(\)*\+,;=]|%[0-9a-f]{2})+@          # combination
diff -u b/core/lib/Drupal/Core/Routing/UrlGenerator.php b/core/lib/Drupal/Core/Routing/UrlGenerator.php
--- b/core/lib/Drupal/Core/Routing/UrlGenerator.php
+++ b/core/lib/Drupal/Core/Routing/UrlGenerator.php
@@ -228,9 +228,8 @@
       // before any / ? or #. Note: we could use url_is_external($path) here, but
       // that would require another function call, and performance inside url() is
       // critical.
-      $scheme_relative = strpos($path, '//') === 0;
       $colonpos = strpos($path, ':');
-      $options['external'] = ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) || $scheme_relative) && Url::stripDangerousProtocols($path) == $path;
+      $options['external'] = ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && Url::stripDangerousProtocols($path) == $path);
     }
 
     if (isset($options['fragment']) && $options['fragment'] !== '') {
@@ -253,8 +252,9 @@
       // contains a ':' before any / ? or #. Note: we could use
       // \Drupal\Component\Utility\UrlHelper::isExternal($path) here, but that
       // would require another function call, and performance here is critical.
+      $scheme_relative = strpos($path, '//') === 0;
       $colonpos = strpos($path, ':');
-      $options['external'] = ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && UrlHelper::stripDangerousProtocols($path) == $path);
+      $options['external'] = ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) || $scheme_relative) && UrlHelper::stripDangerousProtocols($path) == $path;
     }
 
     if (isset($options['fragment']) && $options['fragment'] !== '') {
reverted:
--- b/core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php
+++ a/core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php
@@ -463,20 +463,6 @@
   }
 
   /**
-   * Add a menu link with a scheme-relative url.
-   */
-  public function testMenuSchemeRelative() {
-    $this->drupalLogin($this->big_user);
-
-    // Make a path with query and fragment on.
-    $path = '//drupal.org';
-    $item = $this->addMenuLink(0, $path);
-
-    $this->drupalGet('admin/structure/menu/item/' . $item['mlid'] . '/edit');
-    $this->assertFieldByName('link_path', $path, 'Path is found with scheme-relative URL.');
-  }
-
-  /**
    * Add a menu link using the menu module UI.
    *
    * @param integer $plid Parent menu link id.
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Common/UrlTest.php
+++ a/core/modules/system/lib/Drupal/system/Tests/Common/UrlTest.php
@@ -290,37 +290,4 @@
     $result = url($url, array('query' => $query));
     $this->assertEqual($url . '&' . http_build_query($query, '', '&'), $result, 'External URL query string can be extended with a custom query string in $options.');
   }
-
-  public function testSchemeRelativeUrls() {
-    $test_url = '//drupal.org';
-
-    // Verify scheme-relative URL can contain a fragment.
-    $url = $test_url . '#drupal';
-    $result = url($url);
-    $this->assertEqual($url, $result, 'Scheme-relative URL with fragment works without a fragment in $options.');
-
-    // Verify fragment can be overidden in an external URL.
-    $url = $test_url . '#drupal';
-    $fragment = $this->randomName(10);
-    $result = url($url, array('fragment' => $fragment));
-    $this->assertEqual($test_url . '#' . $fragment, $result, 'Scheme-relative URL fragment is overidden with a custom fragment in $options.');
-
-    // Verify external URL can contain a query string.
-    $url = $test_url . '?drupal=awesome';
-    $result = url($url);
-    $this->assertEqual($url, $result, 'Scheme-relative URL with query string works without a query string in $options.');
-
-    // Verify external URL can be extended with a query string.
-    $url = $test_url;
-    $query = array($this->randomName(5) => $this->randomName(5));
-    $result = url($url, array('query' => $query));
-    $this->assertEqual($url . '?' . http_build_query($query, '', '&'), $result, 'Scheme-relative URL can be extended with a query string in $options.');
-
-    // Verify query string can be extended in an external URL.
-    $url = $test_url . '?drupal=awesome';
-    $query = array($this->randomName(5) => $this->randomName(5));
-    $result = url($url, array('query' => $query));
-    $this->assertEqual($url . '&' . http_build_query($query, '', '&'), $result, 'Scheme-relative URL query string can be extended with a custom query string in $options.');
-  }
-
 }
reverted:
--- b/core/tests/Drupal/Tests/Component/Utility/UrlTest.php
+++ a/core/tests/Drupal/Tests/Component/Utility/UrlTest.php
@@ -96,7 +96,7 @@
    * @dataProvider providerTestValidAbsoluteData
    */
   public function testValidAbsolute($url, $scheme) {
+    $test_url = $scheme . '://' . $url;
-    $test_url = $scheme . $url;
     $valid_url = Url::isValid($test_url, TRUE);
     $this->assertTrue($valid_url, String::format('@url is a valid URL.', array('@url' => $test_url)));
   }
@@ -124,7 +124,7 @@
    * @dataProvider providerTestInvalidAbsolute
    */
   public function testInvalidAbsolute($url, $scheme) {
+    $test_url = $scheme . '://' . $url;
-    $test_url = $scheme . $url;
     $valid_url = Url::isValid($test_url, TRUE);
     $this->assertFalse($valid_url, String::format('@url is NOT a valid URL.', array('@url' => $test_url)));
   }
@@ -198,7 +198,7 @@
    *   A list of provider data with schemes.
    */
   protected function dataEnhanceWithScheme(array $urls) {
+    $url_schemes = array('http', 'https', 'ftp');
-    $url_schemes = array('http://', 'https://', 'ftp://', '//');
     $data = array();
     foreach ($url_schemes as $scheme) {
       foreach ($urls as $url) {
only in patch2:
unchanged:
--- a/core/lib/Drupal/Component/Utility/UrlHelper.php
+++ b/core/lib/Drupal/Component/Utility/UrlHelper.php
@@ -214,10 +214,11 @@ public static function encodePath($path) {
    */
   public static function isExternal($path) {
     $colonpos = strpos($path, ':');
+    $scheme_relative = strpos($path, '//') === 0;
     // Avoid calling stripDangerousProtocols() if there is any
     // slash (/), hash (#) or question_mark (?) before the colon (:)
     // occurrence - if any - as this would clearly mean it is not a URL.
-    return $colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && static::stripDangerousProtocols($path) == $path;
+    return ($colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) || $scheme_relative) && static::stripDangerousProtocols($path) == $path;
   }
 
   /**
@@ -355,7 +356,7 @@ public static function isValid($url, $absolute = FALSE) {
     if ($absolute) {
       return (bool) preg_match("
         /^                                                      # Start at the beginning of the text
-        (?:ftp|https?|feed):\/\/                                # Look for ftp, http, https or feed schemes
+        (?:ftp:|https?:|feed:)?\/\/                             # Look for ftp, http, https, feed or scheme relative schemes
         (?:                                                     # Userinfo (optional) which is typically
           (?:(?:[\w\.\-\+!$&'\(\)*\+,;=]|%[0-9a-f]{2})+:)*      # a username or a username and password
           (?:[\w\.\-\+%!$&'\(\)*\+,;=]|%[0-9a-f]{2})+@          # combination
only in patch2:
unchanged:
--- a/core/modules/menu_ui/src/Tests/MenuTest.php
+++ b/core/modules/menu_ui/src/Tests/MenuTest.php
@@ -587,6 +587,20 @@ public function testBlockContextualLinks() {
   }
 
   /**
+   * Add a menu link with a scheme-relative url.
+   */
+  public function testMenuSchemeRelative() {
+    $this->drupalLogin($this->adminUser);
+
+    // Make a path with query and fragment on.
+    $path = '//drupal.org';
+    $item = $this->addMenuLink(0, $path);
+
+    $this->drupalGet('admin/structure/menu/item/' . $item['mlid'] . '/edit');
+    $this->assertFieldByName('link_path', $path, 'Path is found with scheme-relative URL.');
+  }
+
+  /**
    * Adds a menu link using the UI.
    *
    * @param string $parent
only in patch2:
unchanged:
--- a/core/modules/system/src/Tests/Common/UrlTest.php
+++ b/core/modules/system/src/Tests/Common/UrlTest.php
@@ -279,4 +279,37 @@ function testExternalUrls() {
     $result = Url::fromUri($url, array('query' => $query))->toString();
     $this->assertEqual($url . '&' . http_build_query($query, '', '&'), $result);
   }
+
+  public function testSchemeRelativeUrls() {
+    $test_url = '//drupal.org';
+
+    // Verify scheme-relative URL can contain a fragment.
+    $url = $test_url . '#drupal';
+    $result = \Drupal::url($url);
+    $this->assertEqual($url, $result, 'Scheme-relative URL with fragment works without a fragment in $options.');
+
+    // Verify fragment can be overidden in an external URL.
+    $url = $test_url . '#drupal';
+    $fragment = $this->randomMachineName(10);
+    $result = \Drupal::url($url, array('fragment' => $fragment));
+    $this->assertEqual($test_url . '#' . $fragment, $result, 'Scheme-relative URL fragment is overidden with a custom fragment in $options.');
+
+    // Verify external URL can contain a query string.
+    $url = $test_url . '?drupal=awesome';
+    $result = \Drupal::url($url);
+    $this->assertEqual($url, $result, 'Scheme-relative URL with query string works without a query string in $options.');
+
+    // Verify external URL can be extended with a query string.
+    $url = $test_url;
+    $query = array($this->randomMachineName(5) => $this->randomMachineName(5));
+    $result = \Drupal::url($url, array('query' => $query));
+    $this->assertEqual($url . '?' . http_build_query($query, '', '&'), $result, 'Scheme-relative URL can be extended with a query string in $options.');
+
+    // Verify query string can be extended in an external URL.
+    $url = $test_url . '?drupal=awesome';
+    $query = array($this->randomMachineName(5) => $this->randomMachineName(5));
+    $result = \Drupal::url($url, array('query' => $query));
+    $this->assertEqual($url . '&' . http_build_query($query, '', '&'), $result, 'Scheme-relative URL query string can be extended with a custom query string in $options.');
+  }
+
 }
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
@@ -92,7 +92,7 @@ public function providerTestValidAbsoluteData() {
    *   The scheme to test.
    */
   public function testValidAbsolute($url, $scheme) {
-    $test_url = $scheme . '://' . $url;
+    $test_url = $scheme . $url;
     $valid_url = UrlHelper::isValid($test_url, TRUE);
     $this->assertTrue($valid_url, String::format('@url is a valid URL.', array('@url' => $test_url)));
   }
@@ -123,7 +123,7 @@ public function providerTestInvalidAbsolute() {
    *   The scheme to test.
    */
   public function testInvalidAbsolute($url, $scheme) {
-    $test_url = $scheme . '://' . $url;
+    $test_url = $scheme . $url;
     $valid_url = UrlHelper::isValid($test_url, TRUE);
     $this->assertFalse($valid_url, String::format('@url is NOT a valid URL.', array('@url' => $test_url)));
   }
@@ -445,7 +445,7 @@ public static function providerTestStripDangerousProtocols() {
    *   A list of provider data with schemes.
    */
   protected function dataEnhanceWithScheme(array $urls) {
-    $url_schemes = array('http', 'https', 'ftp');
+    $url_schemes = array('http://', 'https://', 'ftp://', '//');
     $data = array();
     foreach ($url_schemes as $scheme) {
       foreach ($urls as $url) {