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(
@@ -171,7 +169,10 @@
     $this->checkConfigurableLanguageWeight('after re-ordering');
 
     // Remove predefined language.
-    $this->drupalPostForm('admin/config/regional/language/delete/fr', array(), 'Delete');
+    $edit = array(
+      'confirm' => 1,
+    );
+    $this->drupalPostForm('admin/config/regional/language/delete/fr', $edit, 'Delete');
     $this->checkConfigurableLanguageWeight('after deleting a language');
   }
 
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.
@@ -177,14 +177,10 @@
 
     // 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.');
+    $extra_values = '&site_default_language=nl';
+    $this->drupalPostForm(NULL, array(), t('Save configuration'), array(), array(), NULL, $extra_values);
+    $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/LanguageLocaleListTest.php b/core/modules/language/tests/src/Functional/LanguageLocaleListTest.php
--- b/core/modules/language/tests/src/Functional/LanguageLocaleListTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageLocaleListTest.php
@@ -57,11 +57,9 @@
 
     // Get language list displayed in select list.
     $this->drupalGet('fr/admin/config/regional/language/add');
-    $option_elements = $this->xpath('//select[@id="edit-predefined-langcode/option"]');
-    $options = [];
-    foreach ($option_elements as $option_element) {
-      $options[] = $option_element->getText();
-    }
+    $select = $this->xpath('//select[@id="edit-predefined-langcode"]');
+    $select_element = (array) end($select);
+    $options = $select_element['option'];
     // Remove the 'Custom language...' option form the end.
     array_pop($options);
     // Order language list.
diff -u b/core/modules/language/tests/src/Functional/LanguageSelectorTranslatableTest.php b/core/modules/language/tests/src/Functional/LanguageSelectorTranslatableTest.php
--- b/core/modules/language/tests/src/Functional/LanguageSelectorTranslatableTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageSelectorTranslatableTest.php
@@ -82,7 +82,7 @@
     $elements = $this->xpath('//select[@id=:id]//option[@value=:option]', array(':id' => 'edit-settings-user-user-settings-language-langcode', ':option' => 'en'));
 
     // Check that the language text is translated.
-    $this->assertEqual($elements[0]->getText(), $name_translation, 'Checking the option string English is translated to Spanish.');
+    $this->assertEqual((string) $elements[0], $name_translation, 'Checking the option string English is translated to Spanish.');
   }
 
 }
diff -u b/core/modules/language/tests/src/Functional/LanguageSwitchingTest.php b/core/modules/language/tests/src/Functional/LanguageSwitchingTest.php
--- b/core/modules/language/tests/src/Functional/LanguageSwitchingTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageSwitchingTest.php
@@ -73,24 +73,22 @@
 
     // Assert that each list item and anchor element has the appropriate data-
     // attributes.
-    $language_switchers = $this->xpath('//div[@id=:id]/ul/li', array(':id' => 'block-test-language-block'));
+    list($language_switcher) = $this->xpath('//div[@id=:id]', array(':id' => 'block-test-language-block'));
     $list_items = array();
     $anchors = array();
     $labels = array();
-    foreach ($language_switchers as $list_item) {
-      $classes = explode(" ", $list_item->getAttribute('class'));
+    foreach ($language_switcher->ul->li as $list_item) {
+      $classes = explode(" ", (string) $list_item['class']);
       list($langcode) = array_intersect($classes, array('en', 'fr'));
       $list_items[] = array(
         'langcode_class' => $langcode,
-        'data-drupal-link-system-path' => $list_item->getAttribute('data-drupal-link-system-path'),
+        'data-drupal-link-system-path' => (string) $list_item['data-drupal-link-system-path'],
       );
-
-      $link = $list_item->find('xpath', 'a');
       $anchors[] = array(
-        'hreflang' => $link->getAttribute('hreflang'),
-        'data-drupal-link-system-path' => $link->getAttribute('data-drupal-link-system-path'),
+        'hreflang' => (string) $list_item->a['hreflang'],
+        'data-drupal-link-system-path' => (string) $list_item->a['data-drupal-link-system-path'],
       );
-      $labels[] = $link->getText();
+      $labels[] = (string) $list_item->a;
     }
     $expected_list_items = array(
       0 => array('langcode_class' => 'en', 'data-drupal-link-system-path' => 'user/2'),
@@ -126,7 +124,7 @@
     $this->assertText($block_label, 'Language switcher block found.');
 
     // Assert that only the current language is marked as active.
-    $language_switchers = $this->xpath('//div[@id=:id]/ul/li', array(':id' => 'block-test-language-block'));
+    list($language_switcher) = $this->xpath('//div[@id=:id]', array(':id' => 'block-test-language-block'));
     $links = array(
       'active' => array(),
       'inactive' => array(),
@@ -136,8 +134,8 @@
       'inactive' => array(),
     );
     $labels = array();
-    foreach ($language_switchers as $list_item) {
-      $classes = explode(" ", $list_item->getAttribute('class'));
+    foreach ($language_switcher->ul->li as $link) {
+      $classes = explode(" ", (string) $link['class']);
       list($langcode) = array_intersect($classes, array('en', 'fr'));
       if (in_array('is-active', $classes)) {
         $links['active'][] = $langcode;
@@ -145,16 +143,14 @@
       else {
         $links['inactive'][] = $langcode;
       }
-
-      $link = $list_item->find('xpath', 'a');
-      $anchor_classes = explode(" ", $link->getAttribute('class'));
+      $anchor_classes = explode(" ", (string) $link->a['class']);
       if (in_array('is-active', $anchor_classes)) {
         $anchors['active'][] = $langcode;
       }
       else {
         $anchors['inactive'][] = $langcode;
       }
-      $labels[] = $link->getText();
+      $labels[] = (string) $link->a;
     }
     $this->assertIdentical($links, array('active' => array('en'), 'inactive' => array('fr')), 'Only the current language list item is marked as active on the language switcher block.');
     $this->assertIdentical($anchors, array('active' => array('en'), 'inactive' => array('fr')), 'Only the current language anchor is marked as active on the language switcher block.');
@@ -212,7 +208,7 @@
       ':hreflang' => 'en',
     ));
     $english_url = $generator->generateFromRoute('entity.user.canonical', array('user' => 2), array('language' => $languages['en']));
-    $this->assertEqual($english_url, $english_link->getAttribute('href'));
+    $this->assertEqual($english_url, (string) $english_link['href']);
 
     // Verify the Italian URL is correct
     list($italian_link) = $this->xpath('//div[@id=:id]/ul/li/a[@hreflang=:hreflang]', array(
@@ -220,7 +216,7 @@
       ':hreflang' => 'it',
     ));
     $italian_url = $generator->generateFromRoute('entity.user.canonical', array('user' => 2), array('language' => $languages['it']));
-    $this->assertEqual($italian_url, $italian_link->getAttribute('href'));
+    $this->assertEqual($italian_url, (string) $italian_link['href']);
   }
 
   /**
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;
@@ -69,9 +69,9 @@
     // For testing path prefix.
     $langcode = 'zh-hans';
     // For setting browser language preference to 'vi'.
-    $http_header_browser_fallback = array("Accept-Language" => "$langcode_browser_fallback;q=1");
+    $http_header_browser_fallback = array("Accept-Language: $langcode_browser_fallback;q=1");
     // For setting browser language preference to some unknown.
-    $http_header_blah = array("Accept-Language" => "blah;q=1");
+    $http_header_blah = array("Accept-Language: blah;q=1");
 
     // Setup the site languages by installing two languages.
     // Set the default language in order for the translated string to be registered
@@ -107,7 +107,7 @@
     );
     $this->drupalPostForm('admin/config/regional/translate', $search, t('Filter'));
     $textarea = current($this->xpath('//textarea'));
-    $lid = $textarea->getAttribute('name');
+    $lid = (string) $textarea[0]['name'];
     $edit = array(
       $lid => $language_browser_fallback_string,
     );
@@ -119,7 +119,7 @@
     );
     $this->drupalPostForm('admin/config/regional/translate', $search, t('Filter'));
     $textarea = current($this->xpath('//textarea'));
-    $lid = $textarea->getAttribute('name');
+    $lid = (string) $textarea[0]['name'];
     $edit = array(
       $lid => $language_string,
     );
@@ -398,7 +398,7 @@
 
     // Access the front page without specifying any valid URL language prefix
     // and having as browser language preference a non-default language.
-    $http_header = array("Accept-Language" => "$langcode_browser_fallback;q=1");
+    $http_header = array("Accept-Language: $langcode_browser_fallback;q=1");
     $language = new Language(array('id' => ''));
     $this->drupalGet('', array('language' => $language), $http_header);
 
@@ -406,11 +406,11 @@
     // language.
     $args = array(':id' => 'block-test-language-block', ':url' => \Drupal::url('<front>') . $langcode_browser_fallback);
     $fields = $this->xpath('//div[@id=:id]//a[@class="language-link is-active" and starts-with(@href, :url)]', $args);
-    $this->assertSame($fields[0]->getText(), $languages[$langcode_browser_fallback]->getName(), 'The browser language is the URL active language');
+    $this->assertTrue($fields[0] == $languages[$langcode_browser_fallback]->getName(), 'The browser language is the URL active language');
 
     // Check that URLs are rewritten using the given browser language.
     $fields = $this->xpath('//div[@class="site-name"]/a[@rel="home" and @href=:url]', $args);
-    $this->assertSame($fields[0]->getText(), 'Drupal', 'URLs are rewritten using the browser language.');
+    $this->assertTrue($fields[0] == 'Drupal', 'URLs are rewritten using the browser language.');
   }
 
   /**
diff -u b/core/modules/language/tests/src/Functional/LanguageUrlRewritingTest.php b/core/modules/language/tests/src/Functional/LanguageUrlRewritingTest.php
--- b/core/modules/language/tests/src/Functional/LanguageUrlRewritingTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageUrlRewritingTest.php
@@ -90,8 +90,9 @@
     // we can always check the prefixed URL.
     $prefixes = language_negotiation_url_prefixes();
     $stored_prefix = isset($prefixes[$language->getId()]) ? $prefixes[$language->getId()] : $this->randomMachineName();
-    $this->assertNotEqual($stored_prefix, $prefix, $message1);
-    $prefix = $stored_prefix;
+    if ($this->assertNotEqual($stored_prefix, $prefix, $message1)) {
+      $prefix = $stored_prefix;
+    }
 
     $this->drupalGet("$prefix/$path");
     $this->assertResponse(404, $message2);