diff --git a/core/includes/form.inc b/core/includes/form.inc
index 9c72685..a2f4ce6 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -17,6 +17,7 @@
 use Drupal\Core\Template\Attribute;
 use Drupal\Core\Url;
 use Symfony\Component\HttpFoundation\RedirectResponse;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 
 /**
  * Retrieves, populates, and processes a form.
@@ -118,7 +119,7 @@ function form_select_options($element, $choices = NULL) {
     }
     else {
       $key = (string) $key;
-      $empty_choice = $empty_value && $key == '_none';
+      $empty_choice = $empty_value && $key == OptionsWidgetBase::OPTIONS_EMPTY_OPTION;
       if ($value_valid && ((!$value_is_array && (string) $element['#value'] === $key || ($value_is_array && in_array($key, $element['#value']))) || $empty_choice)) {
         $selected = ' selected="selected"';
       }
diff --git a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php
index f33fe2a..ae2933d 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php
@@ -18,6 +18,7 @@
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\Core\Session\AccountInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 
 /**
  * Default plugin implementation of the Entity Reference Selection plugin.
@@ -113,7 +114,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     $selection_handler_settings += array(
       'target_bundles' => array(),
       'sort' => array(
-        'field' => '_none',
+        'field' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION,
       ),
       'auto_create' => FALSE,
     );
@@ -169,7 +170,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
         '#type' => 'select',
         '#title' => $this->t('Sort by'),
         '#options' => array(
-          '_none' => $this->t('- None -'),
+            OptionsWidgetBase::OPTIONS_EMPTY_OPTION => $this->t('- None -'),
         ) + $fields,
         '#ajax' => TRUE,
         '#limit_validation_errors' => array(),
@@ -182,7 +183,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
         '#process' => array('_entity_reference_form_process_merge_parent'),
       );
 
-      if ($selection_handler_settings['sort']['field'] != '_none') {
+      if ($selection_handler_settings['sort']['field'] != OptionsWidgetBase::OPTIONS_EMPTY_OPTION) {
         // Merge-in default values.
         $selection_handler_settings['sort'] += array(
           'direction' => 'ASC',
@@ -345,7 +346,7 @@ protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS')
     // Add the sort option.
     if (!empty($handler_settings['sort'])) {
       $sort_settings = $handler_settings['sort'];
-      if ($sort_settings['field'] != '_none') {
+      if ($sort_settings['field'] != OptionsWidgetBase::OPTIONS_EMPTY_OPTION) {
         $query->sort($sort_settings['field'], $sort_settings['direction']);
       }
     }
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php
index b155c3b..c6b54eb 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php
@@ -36,6 +36,12 @@
    */
   const OPTIONS_EMPTY_SELECT = 'option_select';
 
+
+  /**
+   * Identifies a 'None' option key.
+   */
+  const OPTIONS_EMPTY_OPTION = '_none';
+
   /**
    * Abstract over the actual field columns, to allow different field types to
    * reuse those widgets.
@@ -81,7 +87,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
    *   The form state.
    */
   public static function validateElement(array $element, FormStateInterface $form_state) {
-    if ($element['#required'] && $element['#value'] == '_none') {
+    if ($element['#required'] && $element['#value'] == static::OPTIONS_EMPTY_OPTION) {
       $form_state->setError($element, t('!name field is required.', array('!name' => $element['#title'])));
     }
 
@@ -99,7 +105,7 @@ public static function validateElement(array $element, FormStateInterface $form_
 
     // Filter out the 'none' option. Use a strict comparison, because
     // 0 == 'any string'.
-    $index = array_search('_none', $values, TRUE);
+    $index = array_search(static::OPTIONS_EMPTY_OPTION, $values, TRUE);
     if ($index !== FALSE) {
       unset($values[$index]);
     }
@@ -141,7 +147,7 @@ protected function getOptions(FieldableEntityInterface $entity) {
             break;
         }
 
-        $options = array('_none' => $label) + $options;
+        $options = array(static::OPTIONS_EMPTY_OPTION => $label) + $options;
       }
 
       $module_handler = \Drupal::moduleHandler();
diff --git a/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php b/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php
index e6718c0..47fac1f 100644
--- a/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php
+++ b/core/modules/entity_reference/src/Tests/EntityReferenceAdminTest.php
@@ -10,6 +10,7 @@
 use Drupal\field_ui\Tests\FieldUiTestTrait;
 use Drupal\simpletest\WebTestBase;
 use Drupal\taxonomy\Entity\Vocabulary;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 
 /**
  * Tests for the administrative UI.
@@ -97,7 +98,7 @@ public function testFieldAdminHandler() {
 
     // Test the sort settings.
     // Option 0: no sort.
-    $this->assertFieldByName('field[settings][handler_settings][sort][field]', '_none');
+    $this->assertFieldByName('field[settings][handler_settings][sort][field]', OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->assertNoFieldByName('field[settings][handler_settings][sort][direction]');
     // Option 1: sort by field.
     $this->drupalPostAjaxForm(NULL, array('field[settings][handler_settings][sort][field]' => 'nid'), 'field[settings][handler_settings][sort][field]');
@@ -111,7 +112,7 @@ public function testFieldAdminHandler() {
     $this->assertFieldByXPath("//select[@name='field[settings][handler_settings][sort][field]']/option[@value='body.value']");
 
     // Set back to no sort.
-    $this->drupalPostAjaxForm(NULL, array('field[settings][handler_settings][sort][field]' => '_none'), 'field[settings][handler_settings][sort][field]');
+    $this->drupalPostAjaxForm(NULL, array('field[settings][handler_settings][sort][field]' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION), 'field[settings][handler_settings][sort][field]');
     $this->assertNoFieldByName('field[settings][handler_settings][sort][direction]');
 
     // Third step: confirm.
@@ -147,7 +148,7 @@ public function testFieldAdminHandler() {
     );
     $this->drupalPostForm($bundle_path . '/fields/' . $field_name . '/storage', $edit, t('Save field settings'));
     $this->drupalGet($bundle_path . '/fields/' . $field_name);
-    $this->assertFieldByName('field[settings][handler_settings][filter][type]', '_none');
+    $this->assertFieldByName('field[settings][handler_settings][filter][type]', OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
 
     // Try to select the views handler.
     $edit = array(
diff --git a/core/modules/entity_reference/src/Tests/EntityReferenceFieldDefaultValueTest.php b/core/modules/entity_reference/src/Tests/EntityReferenceFieldDefaultValueTest.php
index c2ceed9..b71f078 100644
--- a/core/modules/entity_reference/src/Tests/EntityReferenceFieldDefaultValueTest.php
+++ b/core/modules/entity_reference/src/Tests/EntityReferenceFieldDefaultValueTest.php
@@ -11,6 +11,7 @@
 use Drupal\config\Tests\SchemaCheckTestTrait;
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\simpletest\WebTestBase;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 
 /**
  * Tests entity reference field default values storage in CMI.
@@ -68,7 +69,7 @@ function testEntityReferenceDefaultValue() {
         'handler' => 'default',
         'handler_settings' => array(
           'target_bundles' => array('referenced_content'),
-          'sort' => array('field' => '_none'),
+          'sort' => array('field' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION),
         ),
       ),
     ));
@@ -131,7 +132,7 @@ function testEntityReferenceDefaultConfigValue() {
       'settings' => array(
         'handler' => 'default',
         'handler_settings' => array(
-          'sort' => array('field' => '_none'),
+          'sort' => array('field' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION),
         ),
       ),
     ));
diff --git a/core/modules/options/options.api.php b/core/modules/options/options.api.php
index c1dd934..3ba6dc9 100644
--- a/core/modules/options/options.api.php
+++ b/core/modules/options/options.api.php
@@ -7,6 +7,7 @@
 
 use Drupal\Core\Entity\FieldableEntityInterface;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 
 /**
  * Alters the list of options to be displayed for a field.
@@ -32,7 +33,7 @@ function hook_options_list_alter(array &$options, array $context) {
   // Check if this is the field we want to change.
   if ($context['field']->id() == 'field_option') {
     // Change the label of the empty option.
-    $options[OPTIONS_EMPTY_NONE] = t('== Empty ==');
+    $options[OptionsWidgetBase::OPTIONS_EMPTY_OPTION] = t('== Empty ==');
   }
 }
 
diff --git a/core/modules/options/options.module b/core/modules/options/options.module
index 78020a0..d221adb 100644
--- a/core/modules/options/options.module
+++ b/core/modules/options/options.module
@@ -12,11 +12,6 @@
 use Drupal\field\FieldStorageConfigInterface;
 
 /**
- * Identifies a 'None' option.
- */
-const OPTIONS_EMPTY_NONE = '_none';
-
-/**
  * Implements hook_help().
  */
 function options_help($route_name, RouteMatchInterface $route_match) {
diff --git a/core/modules/options/src/Tests/OptionsSelectDynamicValuesTest.php b/core/modules/options/src/Tests/OptionsSelectDynamicValuesTest.php
index 1fef8ec..2f422cd 100644
--- a/core/modules/options/src/Tests/OptionsSelectDynamicValuesTest.php
+++ b/core/modules/options/src/Tests/OptionsSelectDynamicValuesTest.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\options\Tests;
 
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
+
 /**
  * Tests an options select with a dynamic allowed values function.
  *
@@ -30,7 +32,7 @@ function testSelectListDynamic() {
     $this->assertEqual(count($options), count($this->test) + 1);
     foreach ($options as $option) {
       $value = (string) $option['value'];
-      if ($value != OPTIONS_EMPTY_NONE) {
+      if ($value != OptionsWidgetBase::OPTIONS_EMPTY_OPTION) {
         $this->assertTrue(array_search($value, $this->test));
       }
     }
diff --git a/core/modules/options/src/Tests/OptionsWidgetsTest.php b/core/modules/options/src/Tests/OptionsWidgetsTest.php
index ae7bc2a..a8f8fd1 100644
--- a/core/modules/options/src/Tests/OptionsWidgetsTest.php
+++ b/core/modules/options/src/Tests/OptionsWidgetsTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\options\Tests;
 
 use Drupal\field\Tests\FieldTestBase;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 
 /**
  * Tests the Options widgets.
@@ -127,7 +128,7 @@ function testRadioButtons() {
     $this->assertNoFieldChecked('edit-card-1-2');
 
     // Unselect option.
-    $edit = array('card_1' => OPTIONS_EMPTY_NONE);
+    $edit = array('card_1' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->drupalPostForm(NULL, $edit, t('Save'));
     $this->assertFieldValues($entity_init, 'card_1', array());
 
@@ -260,14 +261,14 @@ function testSelectListSingle() {
     $this->assertTrue($this->xpath('//select[@id=:id]//option[@value="_none" and text()=:label]', array(':id' => 'edit-card-1', ':label' => t('- Select a value -'))), 'A required select list has a "Select a value" choice.');
 
     // With no field data, nothing is selected.
-    $this->assertNoOptionSelected('edit-card-1', OPTIONS_EMPTY_NONE);
+    $this->assertNoOptionSelected('edit-card-1', OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->assertNoOptionSelected('edit-card-1', 0);
     $this->assertNoOptionSelected('edit-card-1', 1);
     $this->assertNoOptionSelected('edit-card-1', 2);
     $this->assertRaw('Some dangerous & unescaped markup', 'Option text was properly filtered.');
 
     // Submit form: select invalid 'none' option.
-    $edit = array('card_1' => OPTIONS_EMPTY_NONE);
+    $edit = array('card_1' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->drupalPostForm(NULL, $edit, t('Save'));
     $this->assertRaw(t('!title field is required.', array('!title' => $field->getName())), 'Cannot save a required field when selecting "none" from the select list.');
 
@@ -293,7 +294,7 @@ function testSelectListSingle() {
     // A non-required field has a 'none' option.
     $this->assertTrue($this->xpath('//select[@id=:id]//option[@value="_none" and text()=:label]', array(':id' => 'edit-card-1', ':label' => t('- None -'))), 'A non-required select list has a "None" choice.');
     // Submit form: Unselect the option.
-    $edit = array('card_1' => OPTIONS_EMPTY_NONE);
+    $edit = array('card_1' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->drupalPostForm('entity_test/manage/' . $entity->id(), $edit, t('Save'));
     $this->assertFieldValues($entity_init, 'card_1', array());
 
@@ -324,7 +325,7 @@ function testSelectListSingle() {
     $this->assertNoOptionSelected('edit-card-1', 2);
 
     // Submit form: Unselect the option.
-    $edit = array('card_1' => OPTIONS_EMPTY_NONE);
+    $edit = array('card_1' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->drupalPostForm('entity_test/manage/' . $entity->id(), $edit, t('Save'));
     $this->assertFieldValues($entity_init, 'card_1', array());
   }
@@ -355,7 +356,7 @@ function testSelectListMultiple() {
 
     // Display form: with no field data, nothing is selected.
     $this->drupalGet('entity_test/manage/' . $entity->id());
-    $this->assertOptionSelected("edit-card-2", OPTIONS_EMPTY_NONE);
+    $this->assertOptionSelected("edit-card-2", OptionsWidgetBase::OPTIONS_EMPTY_OPTION);
     $this->assertNoOptionSelected('edit-card-2', 0);
     $this->assertNoOptionSelected('edit-card-2', 1);
     $this->assertNoOptionSelected('edit-card-2', 2);
@@ -397,12 +398,12 @@ function testSelectListMultiple() {
 
     // Check that the 'none' option has no effect if actual options are selected
     // as well.
-    $edit = array('card_2[]' => array(OPTIONS_EMPTY_NONE => OPTIONS_EMPTY_NONE, 0 => 0));
+    $edit = array('card_2[]' => array(OptionsWidgetBase::OPTIONS_EMPTY_OPTION => OptionsWidgetBase::OPTIONS_EMPTY_OPTION, 0 => 0));
     $this->drupalPostForm('entity_test/manage/' . $entity->id(), $edit, t('Save'));
     $this->assertFieldValues($entity_init, 'card_2', array(0));
 
     // Check that selecting the 'none' option empties the field.
-    $edit = array('card_2[]' => array(OPTIONS_EMPTY_NONE => OPTIONS_EMPTY_NONE));
+    $edit = array('card_2[]' => array(OptionsWidgetBase::OPTIONS_EMPTY_OPTION => OptionsWidgetBase::OPTIONS_EMPTY_OPTION));
     $this->drupalPostForm('entity_test/manage/' . $entity->id(), $edit, t('Save'));
     $this->assertFieldValues($entity_init, 'card_2', array());
 
@@ -445,7 +446,7 @@ function testSelectListMultiple() {
     $this->assertNoOptionSelected('edit-card-2', 2);
 
     // Submit form: Unselect the option.
-    $edit = array('card_2[]' => array(OPTIONS_EMPTY_NONE => OPTIONS_EMPTY_NONE));
+    $edit = array('card_2[]' => array(OptionsWidgetBase::OPTIONS_EMPTY_OPTION => OptionsWidgetBase::OPTIONS_EMPTY_OPTION));
     $this->drupalPostForm('entity_test/manage/' . $entity->id(), $edit, t('Save'));
     $this->assertFieldValues($entity_init, 'card_2', array());
   }
diff --git a/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
index 3b382e1..6482093 100644
--- a/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
@@ -10,6 +10,7 @@
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 use Drupal\Core\Url;
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\field\Entity\FieldConfig;
@@ -244,7 +245,7 @@ protected function createReferenceTestEntities($referenced_entity) {
           'target_bundles' => array(
             $referenced_entity->bundle() => $referenced_entity->bundle(),
           ),
-          'sort' => array('field' => '_none'),
+          'sort' => array('field' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION),
           'auto_create' => FALSE,
         ),
       ),
diff --git a/core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php b/core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php
index d766378..5fb2043 100644
--- a/core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php
+++ b/core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php
@@ -14,6 +14,7 @@
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase;
 use Drupal\user\RoleInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
@@ -85,7 +86,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     // Merge in default values.
     $selection_handler_settings += array(
       'filter' => array(
-        'type' => '_none',
+        'type' => OptionsWidgetBase::OPTIONS_EMPTY_OPTION,
       ),
       'include_anonymous' => TRUE,
     );
@@ -101,7 +102,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
       '#type' => 'select',
       '#title' => $this->t('Filter by'),
       '#options' => array(
-        '_none' => $this->t('- None -'),
+        OptionsWidgetBase::OPTIONS_EMPTY_OPTION => $this->t('- None -'),
         'role' => $this->t('User role'),
       ),
       '#ajax' => TRUE,