diff --git a/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
index 246fe72..92626d7 100644
--- a/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -19,6 +19,22 @@
 
 /**
  * Defines an object that holds information about a URL.
+ *
+ * This class supports:
+ * - External URLs.
+ * - Internal URLs that correspond to routes handled by Drupal's routing system.
+ * - Base-relative and host-relative URLs not handled by Drupal.
+ * - URLs for user input.
+ * - URLs for entities.
+ *
+ * New URLs should generally be constructed using one of the static factory
+ * methods. See:
+ * - Url::fromRoute()
+ * - Url::fromRouteMatch()
+ * - Url::fromUri()
+ * - Url::fromUserInput()
+ * - Url::fromBaseRelativePath()
+ * - Url::fromHostRelativePath()
  */
 class Url {
 
@@ -28,21 +44,38 @@ class Url {
    * Scheme for base-relative, unrouted URIs.
    *
    * This scheme should only be used for unrouted URIs, like base:robots.txt
-   * or base:update.php. To create such a base-relative URI from a path, use
-   * Url::fromBaseRelativePath().
+   * or base:core/update.php. To create such a base-relative URI from a path,
+   * use Url::fromBaseRelativePath(). To create a host-relative URI instead
+   * (for example, for http://example.com/index.html when Drupal is installed
+   * in http://example.com/drupal/), use Url::fromHostRelativePath().
    *
    * @see static::fromBaseRelativePath()
+   * @see static::fromHostRelativePath()
    * @see static::fromRoute()
    * @see static::fromUri()
    */
   const SCHEME_BASE = 'base';
 
   /**
+   * Scheme for URIs from user input.
+   *
+   * To create a URL for paths from user input that may or may not correspond
+   * to a valid Drupal route, use Url::fromUserInput().
+   *
+   * @see static::fromUserInput()
+   * @see static::fromUserPathUri()
+   *
+   * @todo Rename this constant depending on the outcome of:
+   *   https://www.drupal.org/node/2417567
+   */
+   const SCHEME_USER_PATH = 'user-path';
+
+  /**
    * Scheme for entity URIs.
    *
    * This scheme should be used for entity URIs, which are given in the form
    * entity:{entity_type}/{entity_id}. To get the URL for an entity, generally
-   * you should use \Drupal\core\Entity\Entity::url() rather than creating a
+   * you should use \Drupal\Core\Entity\Entity::url() rather than creating a
    * URI directly.
    *
    * @see static::fromEntityUri()
@@ -52,6 +85,9 @@ class Url {
   /**
    * Scheme for routed URIs.
    *
+   * This scheme is used internally for handling routes as URIs. To get the
+   * URL for a route, use Url::fromRoute().
+   *
    * @see static::fromRouteUri()
    *
    * @todo What exactly is the point of this scheme? See:
@@ -138,8 +174,11 @@ class Url {
    * constructing Url objects directly, in order to avoid ambiguity and make
    * your code more self-documenting:
    * - Url::fromRoute()
+   * - Url::fromRouteMatch()
    * - Url::fromUri()
+   * - Url::fromUserInput()
    * - Url::fromBaseRelativePath()
+   * - Url::fromHostRelativePath()
    *
    * @param string $route_name_or_uri
    *   The name of the route or URI.
@@ -161,10 +200,6 @@ class Url {
    *   - 'https': Whether this URL should point to a secure location. If not
    *     defined, the current scheme is used, so the user stays on HTTP or HTTPS
    *     respectively. TRUE enforces HTTPS and FALSE enforces HTTP.
-   *
-   * @see static::fromRoute()
-   * @see static::fromUri()
-   * @see static::fromBaseRelativePath()
    */
   public function __construct($route_name_or_uri, $route_parameters = array(), $options = array()) {
     $this->routeName = $route_name_or_uri;
@@ -176,8 +211,9 @@ public function __construct($route_name_or_uri, $route_parameters = array(), $op
    * Creates a new Url object for a URL that has a Drupal route.
    *
    * This method is for URLs that have Drupal routes (that is, most pages
-   * generated by Drupal). For non-routed local URIs relative to the base
-   * path (like robots.txt) use Url::fromBaseRelativePath().
+   * generated by Drupal). To generate a URL for a route match instead (that
+   * is, the route corresponding to a particular request), use
+   * Url::fromRouteMatch().
    *
    * @param string $route_name
    *   The name of the route
@@ -203,8 +239,10 @@ public function __construct($route_name_or_uri, $route_parameters = array(), $op
    * @return \Drupal\Core\Url
    *   A new Url object for a routed (internal to Drupal) URL.
    *
-   * @see static::fromUri()
+   * @see static::fromRouteMatch()
+   * @see static::fromUserInput()
    * @see static::fromBaseRelativePath()
+   * @see static::fromUri()
    */
   public static function fromRoute($route_name, $route_parameters = array(), $options = array()) {
     return new static($route_name, $route_parameters, $options);
@@ -215,7 +253,7 @@ public static function fromRoute($route_name, $route_parameters = array(), $opti
    *
    * This method should only be used for paths within the base directory of the
    * site installation that do not have a Drupal route. For example, robots.txt
-   * and update.php are unrouted, base-relative paths.
+   * and core/update.php are unrouted, base-relative paths.
    *
    * Do not use this method for URLs that do have a Drupal route (that is, most
    * pages generated by Drupal) because this will circumvent the routing system.
@@ -229,6 +267,7 @@ public static function fromRoute($route_name, $route_parameters = array(), $opti
    *   A new Url object for an unrouted, base-relative URL not internal to
    *   Drupal, but within the site directory.
    *
+   * @see static::fromHostRelativePath()
    * @see static::fromUri()
    * @see static::fromRoute()
    */
@@ -237,12 +276,50 @@ public static function fromBaseRelativePath($path, $options = array()) {
   }
 
   /**
-   * Creates a new URL object from a route match.
+   * Creates a new URL object from an unrouted, host-relative path.
+   *
+   * This method should only be used for paths within the base directory of the
+   * site installation that do not have a Drupal route. For example, robots.txt
+   * and core/update.php are unrouted, base-relative paths.
+   *
+   * Do not use this method for URLs that do have a Drupal route (that is, most
+   * pages generated by Drupal) because this will circumvent the routing system.
+   *
+   * @param string $path
+   *   The host-relative path. For example, if Drupal is installed in the
+   *   subdirectory http://example.com/drupal/, use 'index.html' for a URL to
+   *   http://example.com/index.html.
+   * @param array $options
+   *   (optional) An array of options. See static::fromUri() for details.
+   *
+   * @return \Drupal\Core\Url
+   *   A new Url object for an unrouted, host-relative URL not internal to
+   *   Drupal, but relative to the site host.
+   *
+   * @see static::fromBaseRelativePath()
+   * @see static::fromUri()
+   * @see static::fromRoute()
+   */
+  public static function fromHostRelativePath($path, $options = array()) {
+    return static::fromUri(static::SCHEME_BASE . ':/' . $path, $options);
+  }
+
+  /**
+   * Creates a new URL object from a route match (e.g. for the current request).
+   *
+   * Use this method to create a URL from a specific route match, that is,
+   * the route that best corresponds to a particular request. This is often
+   * used to generate URLs based on the URL for the current request. To
+   * generate a URL for a specific route (rather than from a request), use
+   * Url::fromRoute().
    *
    * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
    *   The route match.
    *
    * @return $this
+   *
+   * @see static::fromRoute()
+   * @see \Drupal\Core\Routing\RouteMatchInterface
    */
   public static function fromRouteMatch(RouteMatchInterface $route_match) {
     if ($route_match->getRouteObject()) {
@@ -256,24 +333,28 @@ public static function fromRouteMatch(RouteMatchInterface $route_match) {
   /**
    * Creates a new Url object from a URI.
    *
-   * This method is for generating URLs for URIs that:
-   * - do not have Drupal routes: both external URLs and unrouted local URIs,
-   *   like base:robots.txt.
-   * - do have a Drupal route, but have a custom scheme to simplify linking.
-   *   Currently, there is only the entity: scheme (This allows URIs of the
-   *   form entity:{entity_type}/{entity_id}. For example: entity:node/1
-   *   resolves to the entity.node.canonical route with a node parameter of 1.)
+   * This method is used to create URLs for internal and external URIs with a
+   * unified API and storage format.
    *
-   * For URLs that have Drupal routes (that is, most pages generated by Drupal),
-   * use Url::fromRoute().
+   * - For URIs that have Drupal routes (that is, most pages generated by
+   *   Drupal), use Url::fromRoute() whenever possible, to avoid circumventing
+   *   the routing system.
+   * - For paths from user input that may or may not correspond to a valid
+   *   Drupal route, use Url::fromUserInput().
+   * - For base-relative and host-relative paths that are not created by Drupal,
+   *   use Url::fromBaseRelativePath() and Url::fromHostRelativePath(),
+   *   respectively.
+   * - For entity URIs, use \Drupal\core\Entity\EntityInterface::url().
+   *
+   * You can call Url::access() on the returned object to do access checking.
    *
    * @param string $uri
-   *   The URI of the resource including the scheme. For user input that may
-   *   correspond to a Drupal route, use user-path: for the scheme. For paths
-   *   that are known not to be handled by the Drupal routing system (such as
-   *   static files), use base: for the scheme to get a link relative to the
-   *   Drupal base path (like the <base> HTML element). For a link to an entity
-   *   you may use entity:{entity_type}/{entity_id} URIs.
+   *   The URI of the resource including the scheme. Several special internal
+   *   schemes are supported. See the documentation of each for details:
+   *   - Url::SCHEME_BASE
+   *   - Url::SCHEME_ROUTE
+   *   - Url::SCHEME_ENTITY
+   *   - Url::SCHEME_USER_PATH
    * @param array $options
    *   (optional) An associative array of additional URL options, with the
    *   following elements:
@@ -291,12 +372,6 @@ public static function fromRouteMatch(RouteMatchInterface $route_match) {
    *     defined, the current scheme is used, so the user stays on HTTP or HTTPS
    *     respectively. TRUE enforces HTTPS and FALSE enforces HTTP.
    *
-   * Note: the user-path: scheme should be avoided except when processing actual
-   * user input that may or may not correspond to a Drupal route. Normally use
-   * Url::fromRoute() for code linking to any any Drupal page.
-   *
-   * You can call access() on the returned object to do access checking.
-   *
    * @return \Drupal\Core\Url
    *   A new Url object with properties depending on the URI scheme.
    *
@@ -304,9 +379,10 @@ public static function fromRouteMatch(RouteMatchInterface $route_match) {
    *   Thrown when the passed in path has no scheme.
    *
    * @see static::fromRoute()
-   * @see static::fromBaseRelativeUri()
-   *
-   * @todo Update this documentation for the new constants and methods.
+   * @see static::fromUserInput()
+   * @see static::fromBaseRelativePath()
+   * @see static::fromHostRelativePath()
+   * @see \Drupal\Core\Entity\EntityInterface::url()
    */
   public static function fromUri($uri, $options = []) {
     $uri_parts = parse_url($uri);
@@ -314,8 +390,7 @@ public static function fromUri($uri, $options = []) {
       throw new \InvalidArgumentException(String::format('The URI "@uri" is malformed.', ['@uri' => $uri]));
     }
     if (empty($uri_parts['scheme'])) {
-      // @todo Update this exception message for the new constants.
-      throw new \InvalidArgumentException(String::format('The URI "@uri" is invalid. You must use a valid URI scheme. Use base: for items like a static file that needs the base path. Use user-path: for user input without a scheme. Use entity: for referencing the canonical route of a content entity. Use route: for directly representing a route name and parameters.', ['@uri' => $uri]));
+      throw new \InvalidArgumentException(String::format('The URI "@uri" is invalid. You must use a valid URI scheme.', ['@uri' => $uri]));
     }
     $uri_parts += ['path' => ''];
     // Discard empty fragment in $options for consistency with parse_url().
@@ -340,7 +415,7 @@ public static function fromUri($uri, $options = []) {
     if ($uri_parts['scheme'] === static::SCHEME_ENTITY) {
       $url = static::fromEntityUri($uri_parts, $uri_options, $uri);
     }
-    elseif ($uri_parts['scheme'] === 'user-path') {
+    elseif ($uri_parts['scheme'] === static::SCHEME_USER_PATH) {
       $url = static::fromUserPathUri($uri_parts, $uri_options);
     }
     elseif ($uri_parts['scheme'] === static::SCHEME_ROUTE) {
@@ -395,6 +470,9 @@ protected static function fromEntityUri(array $uri_parts, array $options, $uri)
    *
    * @return \Drupal\Core\Url
    *   A new Url object for a 'user-path:' URI.
+   *
+   * @todo Rename this method depending on the outcome of:
+   *   https://www.drupal.org/node/2417567
    */
   protected static function fromUserPathUri(array $uri_parts, array $options) {
     $url = \Drupal::pathValidator()
@@ -498,7 +576,7 @@ protected function setUnrouted() {
    */
   public function toUriString() {
     if ($this->isRouted()) {
-      $uri = static::SCHEME_ROUTE . $this->routeName;
+      $uri = static::SCHEME_ROUTE . ':' . $this->routeName;
       if ($this->routeParameters) {
         $uri .= ';' . UrlHelper::buildQuery($this->routeParameters);
       }