diff --git a/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php
index 6006f87..b8f4cd3 100644
--- a/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/RedirectResponseSubscriber.php
@@ -122,13 +122,15 @@ protected function getDestinationAsAbsoluteUrl($destination, $scheme_and_host) {
         // not yet been converted to URLs (outbound path processors and other
         // URL handling still needs to be performed).
         $destination = UrlHelper::parse($destination);
-        $uri = 'base:' . $destination['path'];
+        $path = '/' . $destination['path'];
         $options = [
           'query' => $destination['query'],
           'fragment' => $destination['fragment'],
           'absolute' => TRUE,
         ];
-        $url = Url::fromUri($uri, $options);
+        // Treat this as if it's user input of a path relative to the site's
+        // base URL.
+        $url = Url::fromUserInput($path, $options);
         $destination = $url->toString();
       }
     }
diff --git a/core/lib/Drupal/Core/Menu/menu.api.php b/core/lib/Drupal/Core/Menu/menu.api.php
index defec19..b37fa61 100644
--- a/core/lib/Drupal/Core/Menu/menu.api.php
+++ b/core/lib/Drupal/Core/Menu/menu.api.php
@@ -452,7 +452,7 @@ function hook_system_breadcrumb_alter(\Drupal\Core\Breadcrumb\Breadcrumb &$bread
  *     URL.
  *   - url: The \Drupal\Core\Url object.
  *   - options: An associative array of additional options that will be passed
- *     to either \Drupal\Core\Routing\UrlGenerator::generateFromPath() or
+ *     to either \Drupal\Core\Utility\UnroutedUrlAssembler::assemble() or
  *     \Drupal\Core\Routing\UrlGenerator::generateFromRoute() to generate the
  *     href attribute for this link, and also used when generating the link.
  *     Defaults to an empty array. It may contain the following elements:
@@ -469,7 +469,7 @@ function hook_system_breadcrumb_alter(\Drupal\Core\Breadcrumb\Breadcrumb &$bread
  *       to work as an argument for the constructor of the class
  *       Drupal\Core\Template\Attribute($options['attributes']).
  *
- * @see \Drupal\Core\Routing\UrlGenerator::generateFromPath()
+ * @see \Drupal\Core\Utility\UnroutedUrlAssembler::assemble()
  * @see \Drupal\Core\Routing\UrlGenerator::generateFromRoute()
  */
 function hook_link_alter(&$variables) {
diff --git a/core/lib/Drupal/Core/PathProcessor/OutboundPathProcessorInterface.php b/core/lib/Drupal/Core/PathProcessor/OutboundPathProcessorInterface.php
index b350112..59f2d0c 100644
--- a/core/lib/Drupal/Core/PathProcessor/OutboundPathProcessorInterface.php
+++ b/core/lib/Drupal/Core/PathProcessor/OutboundPathProcessorInterface.php
@@ -22,7 +22,7 @@
    *   The path to process, with a leading slash.
    * @param array $options
    *   An array of options such as would be passed to the generator's
-   *   generateFromPath() method.
+   *   generateFromRoute() method.
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The HttpRequest object representing the current request.
    * @param \Drupal\Core\Render\BubbleableMetadata $bubbleable_metadata
diff --git a/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php b/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
index 6be8a5a..b4a794e 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
@@ -9,7 +9,9 @@
 
 use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\Form\FormState;
+use Drupal\Core\Routing\UrlGeneratorInterface;
 use Drupal\Core\Url;
+use Drupal\Core\Utility\UnroutedUrlAssemblerInterface;
 use Drupal\Tests\UnitTestCase;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use Symfony\Component\HttpFoundation\Request;
@@ -29,11 +31,19 @@ class FormSubmitterTest extends UnitTestCase {
   protected $urlGenerator;
 
   /**
+   * The mocked unrouted URL assembler.
+   *
+   * @var \PHPUnit_Framework_MockObject_MockObject|\Drupal\Core\Utility\UnroutedUrlAssemblerInterface
+   */
+  protected $unroutedUrlAssembler;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUp() {
     parent::setUp();
-    $this->urlGenerator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');
+    $this->urlGenerator = $this->getMock(UrlGeneratorInterface::class);
+    $this->unroutedUrlAssembler = $this->getMock(UnroutedUrlAssemblerInterface::class);
   }
 
   /**
@@ -187,9 +197,13 @@ public function testRedirectWithResponseObject() {
   public function testRedirectWithoutResult() {
     $form_submitter = $this->getFormSubmitter();
     $this->urlGenerator->expects($this->never())
-      ->method('generateFromPath');
-    $this->urlGenerator->expects($this->never())
       ->method('generateFromRoute');
+    $this->unroutedUrlAssembler->expects($this->never())
+      ->method('assemble');
+    $container = new ContainerBuilder();
+    $container->set('url_generator', $this->urlGenerator);
+    $container->set('unrouted_url_assembler', $this->unroutedUrlAssembler);
+    \Drupal::setContainer($container);
     $form_state = $this->getMock('Drupal\Core\Form\FormStateInterface');
     $form_state->expects($this->once())
       ->method('getRedirect')
diff --git a/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php b/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php
index 6607cb5..4d91218 100644
--- a/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php
+++ b/core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php
@@ -349,79 +349,6 @@ public function testUrlGenerationWithHttpsRequirement() {
   }
 
   /**
-   * Tests path-based URL generation.
-   */
-  public function testPathBasedURLGeneration() {
-    $base_path = '/subdir';
-    $base_url = 'http://www.example.com' . $base_path;
-
-    foreach (array('', 'index.php/') as $script_path) {
-      foreach (array(FALSE, TRUE) as $absolute) {
-        // Setup a fake request which looks like a Drupal installed under the
-        // subdir "subdir" on the domain www.example.com.
-        // To reproduce the values install Drupal like that and use a debugger.
-        $server = [
-          'SCRIPT_NAME' => '/subdir/index.php',
-          'SCRIPT_FILENAME' => $this->root . '/index.php',
-          'SERVER_NAME' => 'http://www.example.com',
-        ];
-        $request = Request::create('/subdir/' . $script_path, 'GET', [], [], [], $server);
-        $request->headers->set('host', ['www.example.com']);
-        $this->requestStack->push($request);
-
-        // Determine the expected bubbleable metadata.
-        $expected_cacheability = (new BubbleableMetadata())
-          ->setCacheContexts($absolute ? ['url.site'] : [])
-          ->setCacheMaxAge(Cache::PERMANENT);
-
-        // Get the expected start of the path string.
-        $base = ($absolute ? $base_url . '/' : $base_path . '/') . $script_path;
-        $url = $base . 'node/123';
-        $result = $this->generator->generateFromPath('node/123', array('absolute' => $absolute));
-        $this->assertEquals($url, $result, "$url == $result");
-        $generated_url = $this->generator->generateFromPath('node/123', array('absolute' => $absolute), TRUE);
-        $this->assertEquals($url, $generated_url->getGeneratedUrl(), "$url == $result");
-        $this->assertEquals($expected_cacheability, BubbleableMetadata::createFromObject($generated_url));
-
-        $url = $base . 'node/123#foo';
-        $result = $this->generator->generateFromPath('node/123', array('fragment' => 'foo', 'absolute' => $absolute));
-        $this->assertEquals($url, $result, "$url == $result");
-        $generated_url = $this->generator->generateFromPath('node/123', array('fragment' => 'foo', 'absolute' => $absolute), TRUE);
-        $this->assertEquals($url, $generated_url->getGeneratedUrl(), "$url == $result");
-        $this->assertEquals($expected_cacheability, BubbleableMetadata::createFromObject($generated_url));
-
-        $url = $base . 'node/123?foo';
-        $result = $this->generator->generateFromPath('node/123', array('query' => array('foo' => NULL), 'absolute' => $absolute));
-        $this->assertEquals($url, $result, "$url == $result");
-        $generated_url = $this->generator->generateFromPath('node/123', array('query' => array('foo' => NULL), 'absolute' => $absolute), TRUE);
-        $this->assertEquals($url, $generated_url->getGeneratedUrl(), "$url == $result");
-        $this->assertEquals($expected_cacheability, BubbleableMetadata::createFromObject($generated_url));
-
-        $url = $base . 'node/123?foo=bar&bar=baz';
-        $result = $this->generator->generateFromPath('node/123', array('query' => array('foo' => 'bar', 'bar' => 'baz'), 'absolute' => $absolute));
-        $this->assertEquals($url, $result, "$url == $result");
-        $generated_url = $this->generator->generateFromPath('node/123', array('query' => array('foo' => 'bar', 'bar' => 'baz'), 'absolute' => $absolute), TRUE);
-        $this->assertEquals($url, $generated_url->getGeneratedUrl(), "$url == $result");
-        $this->assertEquals($expected_cacheability, BubbleableMetadata::createFromObject($generated_url));
-
-        $url = $base . 'node/123?foo#bar';
-        $result = $this->generator->generateFromPath('node/123', array('query' => array('foo' => NULL), 'fragment' => 'bar', 'absolute' => $absolute));
-        $this->assertEquals($url, $result, "$url == $result");
-        $generated_url = $this->generator->generateFromPath('node/123', array('query' => array('foo' => NULL), 'fragment' => 'bar', 'absolute' => $absolute), TRUE);
-        $this->assertEquals($url, $generated_url->getGeneratedUrl(), "$url == $result");
-        $this->assertEquals($expected_cacheability, BubbleableMetadata::createFromObject($generated_url));
-
-        $url = $base;
-        $result = $this->generator->generateFromPath('<front>', array('absolute' => $absolute));
-        $this->assertEquals($url, $result, "$url == $result");
-        $generated_url = $this->generator->generateFromPath('<front>', array('absolute' => $absolute), TRUE);
-        $this->assertEquals($url, $generated_url->getGeneratedUrl(), "$url == $result");
-        $this->assertEquals($expected_cacheability, BubbleableMetadata::createFromObject($generated_url));
-      }
-    }
-  }
-
-  /**
    * Tests generating a relative URL with no path.
    *
    * @param array $options