diff --git a/core/modules/content_translation/content_translation.module b/core/modules/content_translation/content_translation.module
index b506079..bceb815 100644
--- a/core/modules/content_translation/content_translation.module
+++ b/core/modules/content_translation/content_translation.module
@@ -675,9 +675,7 @@ function content_translation_element_info_alter(&$type) {
  */
 function content_translation_enable_widget($entity_type, $bundle, array &$form, FormStateInterface $form_state) {
   $key = $form_state->get(['content_translation', 'key']);
-  if (!$context = $form_state->get(['language', $key])) {
-    $context = [];
-  }
+  $context = $form_state->get(['language', $key]) ?: [];
   $context += ['entity_type' => $entity_type, 'bundle' => $bundle];
   $form_state->set(['language', $key], $context);
   $element = content_translation_language_configuration_element_process(array('#name' => $key), $form_state, $form);
diff --git a/core/modules/system/src/Tests/Form/FormTest.php b/core/modules/system/src/Tests/Form/FormTest.php
index cfcc299..a377e6e 100644
--- a/core/modules/system/src/Tests/Form/FormTest.php
+++ b/core/modules/system/src/Tests/Form/FormTest.php
@@ -112,7 +112,7 @@ function testRequiredFields() {
           $user_input['form_id'] = $form_id;
           $form_state->setUserInput($user_input);
           $form_state->setFormObject(new StubForm($form_id, $form));
-          $form_state->setMethod('post');
+          $form_state->setMethod('POST');
           // The form token CSRF protection should not interfere with this test,
           // so we bypass it by setting the token to FALSE.
           $form['#token'] = FALSE;
diff --git a/core/modules/system/src/Tests/Form/ProgrammaticTest.php b/core/modules/system/src/Tests/Form/ProgrammaticTest.php
index be44907..ef8130c 100644
--- a/core/modules/system/src/Tests/Form/ProgrammaticTest.php
+++ b/core/modules/system/src/Tests/Form/ProgrammaticTest.php
@@ -86,6 +86,7 @@ private function submitForm($values, $valid_input) {
 
     // We check submitted values only if we have a valid input.
     if ($valid_input) {
+      // Fetching the values that were set in the submission handler.
       $stored_values = $form_state->get('programmatic_form_submit');
       foreach ($values as $key => $value) {
         $this->assertEqual($stored_values[$key], $value, format_string('Submission handler correctly executed: %stored_key is %stored_value', array('%stored_key' => $key, '%stored_value' => print_r($value, TRUE))));
diff --git a/core/modules/system/src/Tests/Form/TriggeringElementTest.php b/core/modules/system/src/Tests/Form/TriggeringElementTest.php
index 89bdd94..24cc2d2 100644
--- a/core/modules/system/src/Tests/Form/TriggeringElementTest.php
+++ b/core/modules/system/src/Tests/Form/TriggeringElementTest.php
@@ -10,7 +10,7 @@
 use Drupal\simpletest\WebTestBase;
 
 /**
- * Tests that FAPI correctly determines the triggering_element.
+ * Tests that FAPI correctly determines the triggering element.
  *
  * @group Form
  */
@@ -36,22 +36,22 @@ function testNoButtonInfoInPost() {
     // Ensure submitting a form with no buttons results in no triggering element
     // and the form submit handler not running.
     $this->drupalPostForm($path, $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('There is no clicked button.', '$form_state[\'triggering_element\'] set to NULL.');
+    $this->assertText('There is no clicked button.', '$form_state->getTriggeringElement() set to NULL.');
     $this->assertNoText('Submit handler for form_test_clicked_button executed.', 'Form submit handler did not execute.');
 
     // Ensure submitting a form with one or more submit buttons results in the
     // triggering element being set to the first one the user has access to. An
     // argument with 'r' in it indicates a restricted (#access=FALSE) button.
     $this->drupalPostForm($path . '/s', $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('The clicked button is button1.', '$form_state[\'triggering_element\'] set to only button.');
+    $this->assertText('The clicked button is button1.', '$form_state->getTriggeringElement() set to only button.');
     $this->assertText('Submit handler for form_test_clicked_button executed.', 'Form submit handler executed.');
 
     $this->drupalPostForm($path . '/s/s', $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('The clicked button is button1.', '$form_state[\'triggering_element\'] set to first button.');
+    $this->assertText('The clicked button is button1.', '$form_state->getTriggeringElement() set to first button.');
     $this->assertText('Submit handler for form_test_clicked_button executed.', 'Form submit handler executed.');
 
     $this->drupalPostForm($path . '/rs/s', $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('The clicked button is button2.', '$form_state[\'triggering_element\'] set to first available button.');
+    $this->assertText('The clicked button is button2.', '$form_state->getTriggeringElement() set to first available button.');
     $this->assertText('Submit handler for form_test_clicked_button executed.', 'Form submit handler executed.');
 
     // Ensure submitting a form with buttons of different types results in the
@@ -59,15 +59,15 @@ function testNoButtonInfoInPost() {
     // the FAPI 'button' type, this should result in the submit handler not
     // executing. The types are 's'(ubmit), 'b'(utton), and 'i'(mage_button).
     $this->drupalPostForm($path . '/s/b/i', $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('The clicked button is button1.', '$form_state[\'triggering_element\'] set to first button.');
+    $this->assertText('The clicked button is button1.', '$form_state->getTriggeringElement() set to first button.');
     $this->assertText('Submit handler for form_test_clicked_button executed.', 'Form submit handler executed.');
 
     $this->drupalPostForm($path . '/b/s/i', $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('The clicked button is button1.', '$form_state[\'triggering_element\'] set to first button.');
+    $this->assertText('The clicked button is button1.', '$form_state->getTriggeringElement() set to first button.');
     $this->assertNoText('Submit handler for form_test_clicked_button executed.', 'Form submit handler did not execute.');
 
     $this->drupalPostForm($path . '/i/s/b', $edit, NULL, array(), array(), $form_html_id);
-    $this->assertText('The clicked button is button1.', '$form_state[\'triggering_element\'] set to first button.');
+    $this->assertText('The clicked button is button1.', '$form_state->getTriggeringElement() set to first button.');
     $this->assertText('Submit handler for form_test_clicked_button executed.', 'Form submit handler executed.');
   }
 
@@ -95,7 +95,7 @@ function testAttemptAccessControlBypass() {
     // Do this with both a negative and positive assertion, because negative
     // assertions alone can be brittle. See testNoButtonInfoInPost() for why the
     //triggering element gets set to 'button2'.
-    $this->assertNoText('The clicked button is button1.', '$form_state[\'triggering_element\'] not set to a restricted button.');
-    $this->assertText('The clicked button is button2.', '$form_state[\'triggering_element\'] not set to a restricted button.');
+    $this->assertNoText('The clicked button is button1.', '$form_state->getTriggeringElement() not set to a restricted button.');
+    $this->assertText('The clicked button is button2.', '$form_state->getTriggeringElement() not set to a restricted button.');
   }
 }
diff --git a/core/modules/system/tests/modules/batch_test/src/Form/BatchTestMultiStepForm.php b/core/modules/system/tests/modules/batch_test/src/Form/BatchTestMultiStepForm.php
index 19bf6d3..8d40948 100644
--- a/core/modules/system/tests/modules/batch_test/src/Form/BatchTestMultiStepForm.php
+++ b/core/modules/system/tests/modules/batch_test/src/Form/BatchTestMultiStepForm.php
@@ -60,8 +60,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
     }
 
     if ($step < 2) {
-      $step++;
-      $form_state->set('step', $step);
+      $form_state->set('step', ++$step);
       $form_state->setRebuild();
     }
 
diff --git a/core/modules/user/src/Form/UserLoginForm.php b/core/modules/user/src/Form/UserLoginForm.php
index 3f702d1..e1275a4 100644
--- a/core/modules/user/src/Form/UserLoginForm.php
+++ b/core/modules/user/src/Form/UserLoginForm.php
@@ -184,8 +184,9 @@ public function validateAuthentication(array &$form, FormStateInterface $form_st
         }
       }
       // We are not limited by flood control, so try to authenticate.
-      // Store uid in form state as a flag for self::validateFinal().
-      $form_state->set('uid', $this->userAuth->authenticate($form_state->getValue('name'), $password));
+      // Store $uid in form state as a flag for self::validateFinal().
+      $uid = $this->userAuth->authenticate($form_state->getValue('name'), $password);
+      $form_state->set('uid', $uid);
     }
   }
 
diff --git a/core/modules/user/src/Tests/UserAccountFormFieldsTest.php b/core/modules/user/src/Tests/UserAccountFormFieldsTest.php
index 152a462..df01adc 100644
--- a/core/modules/user/src/Tests/UserAccountFormFieldsTest.php
+++ b/core/modules/user/src/Tests/UserAccountFormFieldsTest.php
@@ -31,8 +31,9 @@ class UserAccountFormFieldsTest extends DrupalUnitTestBase {
   function testInstallConfigureForm() {
     require_once DRUPAL_ROOT . '/core/includes/install.core.inc';
     require_once DRUPAL_ROOT . '/core/includes/install.inc';
+    $install_state = install_state_defaults();
     $form_state = new FormState();
-    $form_state->addBuildInfo('args', [install_state_defaults()]);
+    $form_state->addBuildInfo('args', [&$install_state]);
     $form = $this->container->get('form_builder')
       ->buildForm('Drupal\Core\Installer\Form\SiteConfigureForm', $form_state);
 
diff --git a/core/modules/views_ui/src/ViewUI.php b/core/modules/views_ui/src/ViewUI.php
index f3f22a3..203374c 100644
--- a/core/modules/views_ui/src/ViewUI.php
+++ b/core/modules/views_ui/src/ViewUI.php
@@ -364,7 +364,7 @@ public function getStandardButtons(&$form, FormStateInterface $form_state, $form
     $cancel_submit = function_exists($form_id . '_cancel') ? $form_id . '_cancel' : array($this, 'standardCancel');
     $form['actions']['cancel'] = array(
       '#type' => 'submit',
-      '#value' => !$form_state->has('ok_button') ? t('Cancel') : t('Ok'),
+      '#value' => !$form_state->get('ok_button') ? t('Cancel') : t('Ok'),
       '#submit' => array($cancel_submit),
       '#validate' => array(),
       '#ajax' => array(