diff --git a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
index bc21ea8..03e13c2 100644
--- a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
+++ b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Utility;
 
+use Drupal\Core\Url;
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Config\ConfigFactoryInterface;
@@ -109,9 +110,7 @@ protected function buildLocalUrl($uri, array $options = []) {
     $request = $this->requestStack->getCurrentRequest();
 
     // Remove the base: scheme.
-    // @todo Consider using a class constant for this in
-    //   https://www.drupal.org/node/2417459
-    $uri = substr($uri, 5);
+    $uri = substr($uri, strlen(Url::SCHEME_BASE));
 
     // Allow (outbound) path processing, if needed. A valid use case is the path
     // alias overview form:
diff --git a/core/tests/Drupal/Tests/Core/UrlTest.php b/core/tests/Drupal/Tests/Core/UrlTest.php
index dcc2c66..bf42140 100644
--- a/core/tests/Drupal/Tests/Core/UrlTest.php
+++ b/core/tests/Drupal/Tests/Core/UrlTest.php
@@ -773,6 +773,66 @@ public function testFromRouteUriWithMissingRouteName() {
   }
 
   /**
+   * Tests the fromBaseRelativePath()
+   *
+   * @covers ::fromBaseRelativePath
+   *
+   * @dataProvider providerTestFromBaseRelativePath
+   */
+  public function testFromBaseRelativePath($path, $options, $uri_string) {
+    $url = Url::fromBaseRelativePath($path, $options);
+
+    $this->assertFalse($url->isRouted());
+    $this->assertSame(Url::SCHEME_BASE, parse_url($url->toUriString(), PHP_URL_SCHEME));
+    $this->assertSame($uri_string, $url->toUriString());
+  }
+
+  /**
+   *  Data provider for testing base relative paths.
+   */
+  public function providerTestFromBaseRelativePath() {
+    return [
+      ['robots.txt', [], Url::SCHEME_BASE . ':robots.txt'],
+      ['/robots.txt', [], Url::SCHEME_BASE . ':robots.txt'],
+      ['//robots.txt', [], Url::SCHEME_BASE . ':robots.txt'],
+      ['/blog/cats?foo=bar', [], Url::SCHEME_BASE . ':blog/cats?foo=bar'],
+      ['/blog/cats', ['query'=>['foo' => 'bar']], Url::SCHEME_BASE . ':blog/cats?foo=bar'],
+      ['/blog/cats#foo', [], Url::SCHEME_BASE . ':blog/cats#foo'],
+      ['/blog/cats', ['fragment'=>'foo'], Url::SCHEME_BASE . ':blog/cats#foo'],
+    ];
+  }
+
+  /**
+   * Tests the fromHostRelativePath()
+   *
+   * @covers ::fromHostRelativePath
+   *
+   * @dataProvider providerTestFromHostRelativePath
+   */
+  public function testFromHostRelativePath($path, $options, $uri_string) {
+    $url = Url::fromHostRelativePath($path, $options);
+
+    $this->assertFalse($url->isRouted());
+    $this->assertSame(Url::SCHEME_BASE, parse_url($url->toUriString(), PHP_URL_SCHEME));
+    $this->assertSame($uri_string, $url->toUriString());
+  }
+
+  /**
+   *  Data provider for testing base relative paths.
+   */
+  public function providerTestFromHostRelativePath() {
+    return [
+      ['index.html', [], Url::SCHEME_BASE . ':/index.html'],
+      ['/index.html', [], Url::SCHEME_BASE . ':/index.html'],
+      ['//index.html', [], Url::SCHEME_BASE . ':/index.html'],
+      ['/blog/cats?foo=bar', [], Url::SCHEME_BASE . ':/blog/cats?foo=bar'],
+      ['/blog/cats', ['query'=>['foo' => 'bar']], Url::SCHEME_BASE . ':/blog/cats?foo=bar'],
+      ['/blog/cats#foo', [], Url::SCHEME_BASE . ':/blog/cats#foo'],
+      ['/blog/cats', ['fragment'=>'foo'], Url::SCHEME_BASE . ':/blog/cats#foo'],
+    ];
+  }
+
+  /**
    * Creates a mock access manager for the access tests.
    *
    * @param bool $access