reverted:
--- b/core/modules/language/tests/src/Functional/LanguageBlockSettingsVisibilityTest.php
+++ a/core/modules/language/tests/src/Functional/LanguageBlockSettingsVisibilityTest.php
@@ -18,17 +18,10 @@
     $this->drupalLogin($admin_user);
     $this->drupalPostForm('admin/config/regional/language/add', array('predefined_langcode' => 'hu'), t('Add language'));
     $this->drupalGet('admin/structure/block/add/system_menu_block:admin/stark');
+    $this->assertNoFieldByXPath('//input[@id="edit-visibility-language-langcodes-und"]', NULL, '\'Not specified\' option does not appear at block config, language settings section.');
+    $this->assertNoFieldByXpath('//input[@id="edit-visibility-language-langcodes-zxx"]', NULL, '\'Not applicable\' option does not appear at block config, language settings section.');
+    $this->assertFieldByXPath('//input[@id="edit-visibility-language-langcodes-en"]', NULL, '\'English\' option appears at block config, language settings section.');
+    $this->assertFieldByXpath('//input[@id="edit-visibility-language-langcodes-hu"]', NULL, '\'Hungarian\' option appears at block config, language settings section.');
-
-    // 'Not specified' option does not appear at block config, language settings
-    // section.
-    $this->assertSession()->fieldNotExists('edit-visibility-language-langcodes-und');
-    // 'Not applicable' option does not appear.
-    $this->assertSession()->fieldNotExists('edit-visibility-language-langcodes-zxx');
-
-    // // 'English' option appears..
-    $this->assertSession()->fieldExists('edit-visibility-language-langcodes-en');
-    // // 'Hungarian' option appears..
-    $this->assertSession()->fieldExists('edit-visibility-language-langcodes-hu');
   }
 
 }
diff -u b/core/modules/language/tests/src/Functional/LanguageConfigurationElementTest.php b/core/modules/language/tests/src/Functional/LanguageConfigurationElementTest.php
--- b/core/modules/language/tests/src/Functional/LanguageConfigurationElementTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageConfigurationElementTest.php
@@ -5,8 +5,8 @@
 use Drupal\Core\Language\LanguageInterface;
 use Drupal\language\Entity\ConfigurableLanguage;
 use Drupal\language\Entity\ContentLanguageSettings;
-use Drupal\Tests\BrowserTestBase;
 use Drupal\taxonomy\Entity\Vocabulary;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests the features of the language configuration element field.
diff -u b/core/modules/language/tests/src/Functional/LanguageConfigurationTest.php b/core/modules/language/tests/src/Functional/LanguageConfigurationTest.php
--- b/core/modules/language/tests/src/Functional/LanguageConfigurationTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageConfigurationTest.php
@@ -34,13 +34,11 @@
 
     // Check if the Default English language has no path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    // Default English has no path prefix.
-    $this->assertSession()->fieldValueEquals('prefix[en]', '');
+    $this->assertFieldByXPath('//input[@name="prefix[en]"]', '', 'Default English has no path prefix.');
 
     // Check that Add language is a primary button.
     $this->drupalGet('admin/config/regional/language/add');
-    $button = $this->assertSession()->buttonExists('Add language');
-    $this->assertTrue($button->hasClass('button--primary'));
+    $this->assertFieldByXPath('//input[contains(@class, "button--primary")]', 'Add language', 'Add language is a primary button');
 
     // Add predefined language.
     $edit = array(
@@ -55,10 +53,10 @@
 
     // Check if the Default English language has no path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertSession()->fieldValueEquals('prefix[en]', '');
+    $this->assertFieldByXPath('//input[@name="prefix[en]"]', '', 'Default English has no path prefix.');
     // Check if French has a path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertSession()->fieldValueEquals('prefix[fr]', 'fr');
+    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'fr', 'French has a path prefix.');
 
     // Check if we can change the default language.
     $this->drupalGet('admin/config/regional/language');
@@ -76,17 +74,17 @@
     // Check if a valid language prefix is added after changing the default
     // language.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertSession()->fieldValueEquals('prefix[en]', 'en');
+    $this->assertFieldByXPath('//input[@name="prefix[en]"]', 'en', 'A valid path prefix has been added to the previous default language.');
     // Check if French still has a path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertSession()->fieldValueEquals('prefix[fr]', 'fr');
+    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'fr', 'French still has a path prefix.');
 
     // Check that prefix can be changed.
     $edit = array(
       'prefix[fr]' => 'french',
     );
     $this->drupalPostForm(NULL, $edit, t('Save configuration'));
-    $this->assertSession()->fieldValueEquals('prefix[fr]', 'french');
+    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'french', 'French path prefix has changed.');
 
     // Check that the prefix can be removed.
     $edit = array(
diff -u b/core/modules/language/tests/src/Functional/LanguageCustomLanguageConfigurationTest.php b/core/modules/language/tests/src/Functional/LanguageCustomLanguageConfigurationTest.php
--- b/core/modules/language/tests/src/Functional/LanguageCustomLanguageConfigurationTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageCustomLanguageConfigurationTest.php
@@ -2,9 +2,9 @@
 
 namespace Drupal\Tests\language\Functional;
 
-use Drupal\Tests\BrowserTestBase;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Adds and configures custom languages.
diff -u b/core/modules/language/tests/src/Functional/LanguageListTest.php b/core/modules/language/tests/src/Functional/LanguageListTest.php
--- b/core/modules/language/tests/src/Functional/LanguageListTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageListTest.php
@@ -3,9 +3,9 @@
 namespace Drupal\Tests\language\Functional;
 
 use Drupal\language\Entity\ConfigurableLanguage;
-use Drupal\Tests\BrowserTestBase;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Adds a new language and tests changing its status and the default language.
@@ -23,6 +23,9 @@
 
   /**
    * Functional tests for adding, editing and deleting languages.
+   *
+   * @expectedException \InvalidArgumentException
+   * @expectedExceptionMessage Input "site_default_language" cannot take "nl" as a value (possible values: xx).
    */
   function testLanguageList() {
 
@@ -177,14 +180,9 @@
 
     // Ensure that NL cannot be set default when it's not available.
     $this->drupalGet('admin/config/regional/language');
-    try {
-      $this->drupalPostForm(NULL, array('site_default_language' => 'nl'), t('Save configuration'));
-      $this->fail('Form validation InvalidArgumentException thrown but not caught.');
-    }
-    catch (\InvalidArgumentException $e) {
-      $this->assertEquals($e->getMessage(), 'Input "site_default_language" cannot take "nl" as a value (possible values: xx).');
-    }
-    $this->assertFieldChecked('edit-site-default-language-xx', 'The previous default language still selected.');
+    $this->drupalPostForm(NULL, array('site_default_language' => 'nl'), t('Save configuration'));
+    $this->assertText(t('Selected default language no longer exists.'));
+    $this->assertNoFieldChecked('edit-site-default-language-xx', 'The previous default language got deselected.');
   }
 
   /**
diff -u b/core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php b/core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php
--- b/core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php
@@ -8,9 +8,9 @@
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationSelected;
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationSession;
 use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Tests\BrowserTestBase;
 use Drupal\user\Plugin\LanguageNegotiation\LanguageNegotiationUser;
 use Drupal\user\Plugin\LanguageNegotiation\LanguageNegotiationUserAdmin;
-use Drupal\Tests\BrowserTestBase;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Language\LanguageInterface;
 use Symfony\Component\HttpFoundation\Request;