diff --git a/core/modules/simpletest/src/BrowserTestBase.php b/core/modules/simpletest/src/BrowserTestBase.php
index 681f2de..e531b65 100644
--- a/core/modules/simpletest/src/BrowserTestBase.php
+++ b/core/modules/simpletest/src/BrowserTestBase.php
@@ -9,13 +9,12 @@
 
 use Behat\Mink\Driver\GoutteDriver;
 use Behat\Mink\Element\Element;
-use Behat\Mink\Element\NodeElement;
 use Behat\Mink\Exception\Exception;
 use Behat\Mink\Mink;
 use Behat\Mink\Session;
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\Random;
-use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Database\ConnectionNotDefinedException;
 use Drupal\Core\Database\Database;
@@ -300,7 +299,10 @@ protected function setUp() {
    *   The file path.
    */
   public static function filePreDeleteCallback($path) {
-    chmod($path, 0700);
+    $success = @chmod($path, 0700);
+    if (!$success) {
+      trigger_error("Can not make $path writable whilst cleaning up test directory. The webserver and phpunit are probably not being run by the same user.");
+    }
   }
 
   /**
@@ -487,7 +489,7 @@ protected function drupalCreateUser(array $permissions = array(), $name = NULL)
     $account = entity_create('user', $edit);
     $account->save();
 
-    $this->assertNotNull($account->id(), String::format('User created with name %name and pass %pass', array('%name' => $edit['name'], '%pass' => $edit['pass'])));
+    $this->assertNotNull($account->id(), SafeMarkup::format('User created with name %name and pass %pass', array('%name' => $edit['name'], '%pass' => $edit['pass'])));
     if (!$account->id()) {
       return FALSE;
     }
@@ -541,7 +543,7 @@ protected function drupalCreateRole(array $permissions, $rid = NULL, $name = NUL
     }
     $result = $role->save();
 
-    $this->assertSame($result, SAVED_NEW, String::format('Created role ID @rid with name @name.', array(
+    $this->assertSame($result, SAVED_NEW, SafeMarkup::format('Created role ID @rid with name @name.', array(
       '@name' => var_export($role->label(), TRUE),
       '@rid' => var_export($role->id(), TRUE),
     )));
@@ -553,7 +555,7 @@ protected function drupalCreateRole(array $permissions, $rid = NULL, $name = NUL
         $assigned_permissions = entity_load('user_role', $role->id())->getPermissions();
         $missing_permissions = array_diff($permissions, $assigned_permissions);
         if ($missing_permissions) {
-          $this->fail(String::format('Failed to create permissions: @perms', array('@perms' => implode(', ', $missing_permissions))));
+          $this->fail(SafeMarkup::format('Failed to create permissions: @perms', array('@perms' => implode(', ', $missing_permissions))));
         }
       }
       return $role->id();
@@ -639,7 +641,7 @@ protected function checkPermissions(array $permissions) {
     $valid = TRUE;
     foreach ($permissions as $permission) {
       if (!in_array($permission, $available)) {
-        $this->fail(String::format('Invalid permission %permission.', array('%permission' => $permission)));
+        $this->fail(SafeMarkup::format('Invalid permission %permission.', array('%permission' => $permission)));
         $valid = FALSE;
       }
     }
@@ -687,7 +689,7 @@ protected function drupalLogin(AccountInterface $account) {
 
     // @see BrowserTestBase::drupalUserIsLoggedIn()
     $account->sessionId = $this->getSession()->getCookie(session_name());
-    $this->assertTrue($this->drupalUserIsLoggedIn($account), String::format('User %name successfully logged in.', array('name' => $account->getUsername())));
+    $this->assertTrue($this->drupalUserIsLoggedIn($account), SafeMarkup::format('User %name successfully logged in.', array('name' => $account->getUsername())));
 
     $this->loggedInUser = $account;
     $this->container->get('current_user')->setAccount($account);
@@ -934,7 +936,7 @@ public function installDrupal() {
     if ($modules) {
       $modules = array_unique($modules);
       $success = $container->get('module_installer')->install($modules, TRUE);
-      $this->assertTrue($success, String::format('Enabled modules: %modules', array('%modules' => implode(', ', $modules))));
+      $this->assertTrue($success, SafeMarkup::format('Enabled modules: %modules', array('%modules' => implode(', ', $modules))));
       $this->rebuildContainer();
     }
 
@@ -1046,7 +1048,7 @@ private function changeDatabasePrefix() {
     // Clone the current connection and replace the current prefix.
     $connection_info = Database::getConnectionInfo('default');
     if (is_null($connection_info)) {
-      throw new \InvalidArgumentException('There is no database connection so no tests can be run.');
+      throw new \InvalidArgumentException('There is no database connection so no tests can be run. You must provide a SIMPLETEST_DB environment variable to run PHPUnit based functional tests outside of run-tests.sh.');
     }
     else {
       Database::renameConnection('default', 'simpletest_original_default');
diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index fce7345..743bf08 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -6,8 +6,10 @@
     <ini name="error_reporting" value="32767"/>
     <!-- Do not limit the amount of memory tests take to run. -->
     <ini name="memory_limit" value="-1"/>
-    <env name="SIMPLETEST_BASE_URL" value="http://localhost"/>
-    <env name="SIMPLETEST_DB" value="mysql://username:password@localhost/databasename#table_prefix"/>
+    <env name="SIMPLETEST_BASE_URL" value=""/>
+    <!-- Example SIMPLETEST_BASE_URL value: http://localhost -->
+    <env name="SIMPLETEST_DB" value=""/>
+    <!-- Example SIMPLETEST_DB value: mysql://username:password@localhost/databasename#table_prefix -->
   </php>
   <testsuites>
     <testsuite name="unit">