Update to Drupal 8.0.0 beta 14. For more information, see https://drupal.org/node/2544542

core/lib/Drupal/Core/Session/SessionConfiguration.php

@@ -129,12 +129,6 @@ class SessionConfiguration implements SessionConfigurationInterface {
     }
     else {
       $host = $request->getHost();
-
-      // Strip www. from hostname.
-      if (strpos($host, 'www.') === 0) {
-        $host = substr($host, 4);
-      }
-
       // To maximize compatibility and normalize the behavior across user
       // agents, the cookie domain should start with a dot.
       $cookie_domain = '.' . $host;

core/lib/Drupal/Core/Session/SessionManager.php

@@ -152,7 +152,7 @@ class SessionManager extends NativeSessionStorage implements SessionManagerInter
   /**
    * Forcibly start a PHP session.
    *
-   * @return boolean
+   * @return bool
    *   TRUE if the session is started.
    */
   protected function startNow() {