diff --git a/core/default.settings.php b/core/default.settings.php
index 9160653..ae4391d 100644
--- a/core/default.settings.php
+++ b/core/default.settings.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Drupal global configuration file.
+ * Drupal configuration file.
  *
  * IMPORTANT NOTE:
  * This file may have been set to read-only by the Drupal installation program.
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index a3f4d1e..6e41e61 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -274,7 +274,6 @@ function config_get_config_directory($type = CONFIG_ACTIVE_DIRECTORY) {
  * @see \Symfony\Component\HttpFoundation\Request::getClientIP()
  */
 function drupal_override_server_variables($variables = array()) {
-  // @todo \Drupal::request() is not available when this function is called.
   $request = \Drupal::request();
   $server_vars = $request->server->all();
   // Allow the provided URL to override any existing values in $_SERVER.
@@ -303,10 +302,7 @@ function drupal_override_server_variables($variables = array()) {
   // Replace elements of the $_SERVER array, as appropriate.
   $request->server->replace($variables + $server_vars + $defaults);
 
-  // @todo Inject a global Request object for all CLI executions + use the
-  //   manipulated $request->server as basis for any subsequently instantiated
-  //   Request objects. Alternatively, sub-class Request or fix it upstream to
-  //   account for CLI environments.
+  // @todo remove once conf_path() no longer uses $_SERVER.
   $_SERVER = $request->server->all();
 }
 
@@ -345,7 +341,7 @@ function drupal_environment_initialize() {
   error_reporting(E_STRICT | E_ALL | error_reporting());
 
   // Override PHP settings required for Drupal to work properly.
-  // default.settings.php contains more runtime settings.
+  // settings.php contains more runtime settings.
   // The .htaccess file contains settings that cannot be changed at runtime.
 
   // Deny execution with enabled "magic quotes" (both GPC and runtime).
@@ -391,11 +387,10 @@ function drupal_settings_initialize() {
   $config = array();
 
   // Read the global /settings.php file.
-  // Exclude it for test requests to prevent settings of the parent site/
-  // test-runner to leak into the test environment.
+  // Exclude it for test requests to prevent settings of the test runner from
+  // leaking into the test environment.
   if (!drupal_valid_test_ua() && is_readable(DRUPAL_ROOT . '/settings.php')) {
     require DRUPAL_ROOT . '/settings.php';
-
   }
 
   // Discover the site directory.
@@ -407,7 +402,6 @@ function drupal_settings_initialize() {
   if ($conf_path !== '' && is_readable(DRUPAL_ROOT . '/' . $conf_path . '/settings.php')) {
     require DRUPAL_ROOT . '/' . $conf_path . '/settings.php';
   }
-
   // Initialize Settings.
   new Settings($settings);
 }
@@ -425,7 +419,6 @@ function _drupal_request_initialize() {
   global $base_path, $base_root, $script_path;
   global $base_secure_url, $base_insecure_url;
 
-
   $is_https = isset($_SERVER['HTTPS']) && strtolower($_SERVER['HTTPS']) == 'on';
 
   if (isset($base_url)) {
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index cbf42d6..85df072 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -271,13 +271,6 @@ function install_begin_request(&$install_state) {
     drupal_override_server_variables($install_state['server']);
   }
 
-  // Initialize the Site directory.
-  // This primes the site path to be used during installation, to allow a bare
-  // site folder to be prepared in the /sites directory, which will be used for
-  // installing Drupal.
-  require_once DRUPAL_ROOT . '/core/lib/Drupal/Core/Utility/Site.php';
-  Site::initInstaller(DRUPAL_ROOT);
-
   // If the hash salt leaks, it becomes possible to forge a valid testing user
   // agent, install a new copy of Drupal, and take over the original site.
   // The user agent header is used to pass a database prefix in the request when
@@ -288,6 +281,12 @@ function install_begin_request(&$install_state) {
     exit;
   }
 
+  // Initialize the Site directory.
+  // This primes the site path to be used during installation, to allow a bare
+  // site folder to be prepared in the /sites directory, which will be used for
+  // installing Drupal.
+  Site::initInstaller(DRUPAL_ROOT);
+
   drupal_bootstrap(DRUPAL_BOOTSTRAP_CONFIGURATION);
 
   // Ensure that procedural dependencies are loaded as early as possible,
diff --git a/core/lib/Drupal/Core/StreamWrapper/PublicStream.php b/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
index cbb596c..2ea5d8c 100644
--- a/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
+++ b/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
@@ -6,6 +6,7 @@
  */
 
 namespace Drupal\Core\StreamWrapper;
+
 use Drupal\Core\Utility\Site;
 
 /**
diff --git a/core/lib/Drupal/Core/SystemListing.php b/core/lib/Drupal/Core/SystemListing.php
index 6d895a2..b4509a3 100644
--- a/core/lib/Drupal/Core/SystemListing.php
+++ b/core/lib/Drupal/Core/SystemListing.php
@@ -6,6 +6,7 @@
  */
 
 namespace Drupal\Core;
+
 use Drupal\Core\Utility\Site;
 
 /**
diff --git a/core/lib/Drupal/Core/SystemListingInfo.php b/core/lib/Drupal/Core/SystemListingInfo.php
index 3575adc..d302699 100644
--- a/core/lib/Drupal/Core/SystemListingInfo.php
+++ b/core/lib/Drupal/Core/SystemListingInfo.php
@@ -28,7 +28,6 @@ protected function profiles($directory) {
     // For SimpleTest to be able to test modules packaged together with a
     // distribution we need to include the profile of the parent site (in
     // which test runs are triggered).
-    // @todo !drupal_installation_attempted() defeats the whole purpose?
     if (drupal_valid_test_ua() && !drupal_installation_attempted()) {
       $testing_profile = \Drupal::config('simpletest.settings')->get('parent_profile');
       if ($testing_profile && $testing_profile != $profile) {
diff --git a/core/lib/Drupal/Core/Utility/Site.php b/core/lib/Drupal/Core/Utility/Site.php
index 6711562..6c59f82 100644
--- a/core/lib/Drupal/Core/Utility/Site.php
+++ b/core/lib/Drupal/Core/Utility/Site.php
@@ -45,7 +45,7 @@ class Site {
    *
    * @var \Drupal\Core\Utility\Site
    */
-  private $original;
+  private static $original;
 
   /**
    * The Site singleton instance.
@@ -68,18 +68,21 @@ class Site {
    * @see drupal_settings_initialize()
    */
   public static function init($root_directory, array $sites = NULL, $custom_path = NULL) {
-    if (!isset(self::$instance)) {
-      new self($root_directory);
-    }
     // Only the installer environment is allowed instantiate the Site singleton
     // prior to drupal_settings_initialize().
     // @see initInstaller()
-    elseif (!self::$instance->isInstaller()) {
-      throw new \BadMethodCallException('Site path is initialized already.');
+    if (isset(self::$instance)) {
+      if (!self::$instance->isInstaller()) {
+        throw new \BadMethodCallException('Site path is initialized already.');
+      }
+      else {
+        // Disable the $isInstaller flag to prevent init() from being invoked
+        // more than once.
+        self::$instance->isInstaller = FALSE;
+      }
     }
-    // Force-override the site directory in tests.
-    if ($test_prefix = drupal_valid_test_ua()) {
-      $custom_path = 'sites/simpletest/' . substr($test_prefix, 10);
+    else {
+      new self($root_directory);
     }
     self::$instance->initializePath($sites, $custom_path);
   }
@@ -98,68 +101,54 @@ public static function init($root_directory, array $sites = NULL, $custom_path =
    * @see install_begin_request()
    */
   public static function initInstaller($root_directory) {
-    // WebTestBase::setUp() invokes Site::setUpTest() prior to invoking the
-    // non-interactive installer, in order to override the singleton and prime
-    // a custom site directory.
-    if (drupal_valid_test_ua()) {
-      return;
-    }
     if (isset(self::$instance)) {
       throw new \BadMethodCallException('Site path is initialized already.');
     }
     // Set a global state flag to denote that we are operating in the special
-    // installer environment. initializePath() is unnecessary, as init() will be
-    // called by drupal_settings_initialize(), possibly passing new values from
-    // the newly created settings.php file.
+    // installer environment.
     new self($root_directory, TRUE);
+    self::$instance->initializePath();
   }
 
   /**
-   * Re-initializes the Site singleton for a test run.
-   *
-   * Called by WebTestBase::setUp() prior to invoking the non-interactive
-   * installer for a test.
+   * Constructs the Site singleton.
+   */
+  private function __construct($root_directory, $is_installer = FALSE) {
+    if (isset(self::$instance)/* && !self::$instance->isInstaller()*/) {
+      throw new \BadMethodCallException('Site path is initialized already.');
+    }
+    $this->root = $root_directory;
+    $this->isInstaller = $is_installer;
+    self::$instance = $this;
+  }
+
+  /**
+   * Re-initializes (resets) the Site singleton for a test run.
    *
-   * @param string $root_directory
-   *   The root directory to use for absolute paths; i.e., DRUPAL_ROOT.
-   * @param string $custom_path
-   *   The relative site directory path to use during the test run; e.g.,
-   *   'sites/simpletest/123456'.
+   * @see \Drupal\simpletest\TestBase::prepareEnvironment()
    */
-  public static function setUpTest($root_directory, $custom_path) {
+  public static function setUpTest() {
+    if (!isset(self::$instance)) {
+      throw new \RuntimeException('No original Site to backup. Missing invocation of Site::init()?');
+    }
     if (!drupal_valid_test_ua()) {
       throw new \BadMethodCallException('Site is not executing a test.');
     }
-    if (!file_exists($root_directory . '/' . $custom_path)) {
-      throw new \InvalidArgumentException("Test site directory '$custom_path' does not exist.");
-    }
-    new self($root_directory, TRUE, $custom_path);
+    self::$original = clone self::$instance;
+    self::$instance = NULL;
   }
 
   /**
    * Reverts the Site singleton to the original after a test run.
+   *
+   * @see \Drupal\simpletest\TestBase::restoreEnvironment()
    */
   public static function tearDownTest() {
-    if (!isset(self::$instance->original)) {
+    if (!isset(self::$original)) {
       throw new \RuntimeException('No original Site to revert to. Missing invocation of Site::setUpTest()?');
     }
-    self::$instance = self::$instance->original;
-  }
-
-  /**
-   * Constructs the Site singleton.
-   */
-  private function __construct($root_directory, $is_installer = FALSE, $custom_path = NULL) {
-    if (isset(self::$instance->original)) {
-      throw new \RuntimeException('Site is overridden for a test already. Duplicate invocation of Site::setUpTest()?');
-    }
-    if (isset(self::$instance)) {
-      $this->original = clone self::$instance;
-    }
-    $this->root = $root_directory;
-    $this->isInstaller = $is_installer;
-    $this->path = $custom_path;
-    self::$instance = $this;
+    self::$instance = clone self::$original;
+    self::$original = NULL;
   }
 
   /**
@@ -180,12 +169,9 @@ private function isInstaller() {
    *   (optional) An explicit site path to set; skipping site negotiation.
    */
   private function initializePath(array $sites = NULL, $custom_path = NULL) {
-    // When executing the non-interactive installer in the parent site/
-    // test-runner, WebTestBase::setUp() primes the site directory path via
-    // Site::setUpTest() already.
-    // @todo Remove this case by adding a 'site' parameter to drupal_install().
-    if (isset($this->path)) {
-      return;
+    // Force-override the site directory in tests.
+    if ($test_prefix = drupal_valid_test_ua()) {
+      $custom_path = 'sites/simpletest/' . substr($test_prefix, 10);
     }
 
     // An explicitly defined $conf_path in /settings.php takes precedence.
@@ -278,6 +264,14 @@ private function determinePath(array $sites, $require_settings) {
   /**
    * Prefixes a given filepath with the site directory, if any.
    *
+   * Ensures that a given filepath does not result in an absolute filesystem
+   * path in case of a string concatenation like the following:
+   * @code
+   * // If $site_path is empty (Drupal's root directory), then the resulting
+   * // filesystem path would become absolute; e.g.: "/some/file"
+   * unlink($site_path . '/' . $some_file);
+   * @endcode
+   *
    * @param string $filepath
    *   The filepath to prefix.
    *
@@ -285,6 +279,7 @@ private function determinePath(array $sites, $require_settings) {
    *   The prefixed filepath.
    */
   private function resolvePath($filepath) {
+    // @todo Remove this convenience/sanity check?
     if ($filepath !== '' && $filepath[0] === '/') {
       $filepath = substr($filepath, 1);
     }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index c703d8d..26692bc 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -1072,8 +1072,11 @@ private function prepareEnvironment() {
 
     // After preparing the environment and changing the database prefix, we are
     // in a valid test environment.
+    if (!is_dir(DRUPAL_ROOT . '/' . $this->siteDirectory)) {
+      throw new \RuntimeException("Test site directory '$this->siteDirectory' does not exist.");
+    }
     drupal_valid_test_ua($this->databasePrefix);
-    Site::setUpTest(DRUPAL_ROOT, $this->siteDirectory);
+    Site::setUpTest();
 
     drupal_set_time_limit($this->timeLimit);
   }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index aeea83d..afa9d12 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -9,6 +9,7 @@
 
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\NestedArray;
+use Drupal\Component\Utility\Settings;
 use Drupal\Component\Utility\String;
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Database\Database;
@@ -17,7 +18,6 @@
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Session\UserSession;
 use Drupal\Core\StreamWrapper\PublicStream;
-use Drupal\Core\Utility\Site;
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\block\Entity\Block;
 use Symfony\Component\HttpFoundation\Request;
@@ -775,7 +775,7 @@ protected function setUp() {
     // Copy and prepare an actual settings.php, so as to resemble a regular
     // installation.
     // Not using File API; a potential error must trigger a PHP warning.
-    copy(DRUPAL_ROOT . '/sites/default/default.settings.php', DRUPAL_ROOT . '/' . $this->siteDirectory . '/settings.php');
+    copy(DRUPAL_ROOT . '/core/default.settings.php', DRUPAL_ROOT . '/' . $this->siteDirectory . '/settings.php');
 
     // All file system paths are created by System module during installation.
     // @see system_requirements()
@@ -786,7 +786,7 @@ protected function setUp() {
     );
     // Add the parent profile's search path to the child site's search paths.
     // @see drupal_system_listing()
-    $settings['conf']['simpletest.settings']['parent_profile'] = (object) array(
+    $settings['config']['simpletest.settings']['parent_profile'] = (object) array(
       'value' => $this->originalProfile,
       'required' => TRUE,
     );
@@ -795,13 +795,16 @@ protected function setUp() {
     // Since Drupal is bootstrapped already, install_begin_request() will not
     // bootstrap into DRUPAL_BOOTSTRAP_CONFIGURATION (again). Hence, we have to
     // reload the newly written custom settings.php manually.
-    drupal_settings_initialize();
+    require DRUPAL_ROOT . '/' . $this->siteDirectory . '/settings.php';
+    new Settings($settings);
+    $GLOBALS['config'] = $config;
 
     // Execute the non-interactive installer.
     require_once DRUPAL_ROOT . '/core/includes/install.core.inc';
     install_drupal($parameters);
 
     // Import new settings.php written by the installer.
+    // @todo Fix non-interactive installer; this should happen automatically.
     drupal_settings_initialize();
     foreach ($GLOBALS['config_directories'] as $type => $path) {
       $this->configDirectories[$type] = $path;
@@ -883,7 +886,6 @@ protected function setUp() {
    * @see install_state_defaults()
    */
   protected function installParameters() {
-    // The prefix is prepared in TestBase::changeDatabasePrefix() already.
     $connection_info = Database::getConnectionInfo();
     $driver = $connection_info['default']['driver'];
     unset($connection_info['default']['driver']);
@@ -1065,7 +1067,6 @@ protected function tearDown() {
       $this->kernel->shutdown();
     }
     parent::tearDown();
-    Site::tearDownTest();
 
     // Ensure that internal logged in variable and cURL options are reset.
     $this->loggedInUser = FALSE;
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/DirectoryTest.php b/core/modules/system/lib/Drupal/system/Tests/File/DirectoryTest.php
index 76ce11f..224d8f5 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/DirectoryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/DirectoryTest.php
@@ -6,6 +6,7 @@
  */
 
 namespace Drupal\system\Tests\File;
+
 use Drupal\Core\Utility\Site;
 
 /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php b/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php
index 656e47d..9a2bfa7 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php
@@ -6,6 +6,7 @@
  */
 
 namespace Drupal\system\Tests\File;
+
 use Drupal\Core\Utility\Site;
 
 /**
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index a2ecfe2..6d0ab12 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -230,7 +230,7 @@ function system_requirements($phase) {
   // Test configuration files and directory for writability.
   if ($phase == 'runtime') {
     $conf_errors = array();
-    $conf_path = Site::getPath();
+    $conf_path = Site::getPath(); # @todo
     if (!drupal_verify_install_file($conf_path, FILE_NOT_WRITABLE, 'dir')) {
       $conf_errors[] = t("The directory %file is not protected from modifications and poses a security risk. You must change the directory's permissions to be non-writable.", array('%file' => $conf_path));
     }