diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 38569ad..c9ad213 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2444,7 +2444,7 @@ function template_preprocess_field_multiple_value_form(&$variables) {
         $items[] = &$element[$key];
       }
     }
-    usort($items, '_field_sort_items_value_helper');
+    usort($items, '_field_multiple_value_form_sort_helper');
 
     // Add the items as table rows.
     foreach ($items as $item) {
@@ -2494,6 +2494,17 @@ function template_preprocess_field_multiple_value_form(&$variables) {
 }
 
 /**
+ * Callback for usort() within field-multiple-value-form.html.twig.
+ *
+ * Sorts using ['_weight']['#value']
+ */
+function _field_multiple_value_form_sort_helper($a, $b) {
+  $a_weight = (is_array($a) && isset($a['_weight']['#value']) ? $a['_weight']['#value'] : 0);
+  $b_weight = (is_array($b) && isset($b['_weight']['#value']) ? $b['_weight']['#value'] : 0);
+  return $a_weight - $b_weight;
+}
+
+/**
  * Provides theme registration for themes across .inc files.
  */
 function drupal_common_theme() {
diff --git a/core/lib/Drupal/Core/Entity/Display/EntityFormDisplayInterface.php b/core/lib/Drupal/Core/Entity/Display/EntityFormDisplayInterface.php
index 28a5aa3..d502c7e 100644
--- a/core/lib/Drupal/Core/Entity/Display/EntityFormDisplayInterface.php
+++ b/core/lib/Drupal/Core/Entity/Display/EntityFormDisplayInterface.php
@@ -89,7 +89,8 @@
    * @endcode
    *
    * Additionally, some processing data is placed in $form_state, and can be
-   * accessed by field_form_get_state() and field_form_set_state().
+   * accessed by \Drupal\Core\Field\WidgetBaseInterface::getWidgetState() and
+   * \Drupal\Core\Field\WidgetBaseInterface::setWidgetState().
    *
    * @param \Drupal\Core\Entity\ContentEntityInterface $entity
    *   The entity.
diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index b366cd4..25898d0 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -63,12 +63,12 @@ public function form(FieldItemListInterface $items, array &$form, array &$form_s
     $parents = $form['#parents'];
 
     // Store field information in $form_state.
-    if (!field_form_get_state($parents, $field_name, $form_state)) {
+    if (!static::getWidgetState($parents, $field_name, $form_state)) {
       $field_state = array(
         'items_count' => count($items),
         'array_parents' => array(),
       );
-      field_form_set_state($parents, $field_name, $form_state, $field_state);
+      static::setWidgetState($parents, $field_name, $form_state, $field_state);
     }
 
     // Collect widget elements.
@@ -106,9 +106,9 @@ public function form(FieldItemListInterface $items, array &$form, array &$form_s
     }
 
     // Populate the 'array_parents' information in $form_state['field'] after
-    // the form is built, so that we catch changes in the form structure performed
-    // in alter() hooks.
-    $elements['#after_build'][] = 'field_form_element_after_build';
+    // the form is built, so that we catch changes in the form structure
+    // performed in alter() hooks.
+    $elements['#after_build'][] = array(get_class($this), 'afterBuild');
     $elements['#field_name'] = $field_name;
     $elements['#field_parents'] = $parents;
     // Enforce the structure of submitted values.
@@ -148,7 +148,7 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
     // Determine the number of widgets to display.
     switch ($cardinality) {
       case FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED:
-        $field_state = field_form_get_state($parents, $field_name, $form_state);
+        $field_state = static::getWidgetState($parents, $field_name, $form_state);
         $max = $field_state['items_count'];
         $is_multiple = TRUE;
         break;
@@ -233,6 +233,23 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
   }
 
   /**
+   * After-build handler for field elements in a form.
+   *
+   * This stores the final location of the field within the form structure so
+   * that flagErrors() can assign validation errors to the right form element.
+   */
+  public static function afterBuild(array $element, array &$form_state) {
+    $parents = $element['#field_parents'];
+    $field_name = $element['#field_name'];
+
+    $field_state = static::getWidgetState($parents, $field_name, $form_state);
+    $field_state['array_parents'] = $element['#array_parents'];
+    static::setWidgetState($parents, $field_name, $form_state, $field_state);
+
+    return $element;
+  }
+
+  /**
    * Submission handler for the "Add another item" button.
    */
   public static function addMoreSubmit(array $form, array &$form_state) {
@@ -244,9 +261,9 @@ public static function addMoreSubmit(array $form, array &$form_state) {
     $parents = $element['#field_parents'];
 
     // Increment the items count.
-    $field_state = field_form_get_state($parents, $field_name, $form_state);
+    $field_state = static::getWidgetState($parents, $field_name, $form_state);
     $field_state['items_count']++;
-    field_form_set_state($parents, $field_name, $form_state, $field_state);
+    static::setWidgetState($parents, $field_name, $form_state, $field_state);
 
     $form_state['rebuild'] = TRUE;
   }
@@ -348,12 +365,12 @@ public function extractFormValues(FieldItemListInterface $items, array $form, ar
       $items->filterEmptyItems();
 
       // Put delta mapping in $form_state, so that flagErrors() can use it.
-      $field_state = field_form_get_state($form['#parents'], $field_name, $form_state);
+      $field_state = static::getWidgetState($form['#parents'], $field_name, $form_state);
       foreach ($items as $delta => $item) {
         $field_state['original_deltas'][$delta] = isset($item->_original_delta) ? $item->_original_delta : $delta;
         unset($item->_original_delta, $item->_weight);
       }
-      field_form_set_state($form['#parents'], $field_name, $form_state, $field_state);
+      static::setWidgetState($form['#parents'], $field_name, $form_state, $field_state);
     }
   }
 
@@ -363,7 +380,7 @@ public function extractFormValues(FieldItemListInterface $items, array $form, ar
   public function flagErrors(FieldItemListInterface $items, ConstraintViolationListInterface $violations, array $form, array &$form_state) {
     $field_name = $this->fieldDefinition->getName();
 
-    $field_state = field_form_get_state($form['#parents'], $field_name, $form_state);
+    $field_state = static::getWidgetState($form['#parents'], $field_name, $form_state);
 
     if ($violations->count()) {
       $form_builder = \Drupal::formBuilder();
@@ -429,6 +446,38 @@ public function flagErrors(FieldItemListInterface $items, ConstraintViolationLis
   /**
    * {@inheritdoc}
    */
+  public static function getWidgetState(array $parents, $field_name, array &$form_state) {
+    return NestedArray::getValue($form_state, static::widgetStateParents($parents, $field_name));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function setWidgetState(array $parents, $field_name, array &$form_state, array $field_state) {
+    NestedArray::setValue($form_state, static::widgetStateParents($parents, $field_name), $field_state);
+  }
+
+  /**
+   * Returns the location of processing information within $form_state.
+   *
+   * @param array $parents
+   *   The array of #parents where the widget lives in the form.
+   * @param string $field_name
+   *   The field name.
+   *
+   * @return array
+   *   The location of processing information within $form_state.
+   */
+  protected static function widgetStateParents(array $parents, $field_name) {
+    // Field processing data is placed at
+    // $form_state['field']['#parents'][...$parents...]['#fields'][$field_name],
+    // to avoid clashes between field names and $parents parts.
+    return array_merge(array('field', '#parents'), $parents, array('#fields', $field_name));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function settingsForm(array $form, array &$form_state) {
     return array();
   }
diff --git a/core/lib/Drupal/Core/Field/WidgetBaseInterface.php b/core/lib/Drupal/Core/Field/WidgetBaseInterface.php
index 3af0549..11e37c0 100644
--- a/core/lib/Drupal/Core/Field/WidgetBaseInterface.php
+++ b/core/lib/Drupal/Core/Field/WidgetBaseInterface.php
@@ -13,9 +13,9 @@
  * Base interface definition for "Field widget" plugins.
  *
  * This interface details base wrapping methods that most widget implementations
- * will want to directly inherit from Drupal\Core\Field\WidgetBase.
- * See Drupal\Core\Field\WidgetInterface for methods that will more
- * likely be overriden.
+ * will want to directly inherit from Drupal\Core\Field\WidgetBase. See
+ * Drupal\Core\Field\WidgetInterface for methods that will more likely be
+ * overriden in actual widget implementations.
  */
 interface WidgetBaseInterface extends PluginSettingsInterface {
 
@@ -71,4 +71,43 @@ public function extractFormValues(FieldItemListInterface $items, array $form, ar
    */
   public function flagErrors(FieldItemListInterface $items, ConstraintViolationListInterface $violations, array $form, array &$form_state);
 
+  /**
+   * Retrieves processing information about the widget from $form_state.
+   *
+   * This method is static so that is can be used in static Form API callbacks.
+   *
+   * @param array $parents
+   *   The array of #parents where the field lives in the form.
+   * @param string $field_name
+   *   The field name.
+   * @param array $form_state
+   *   The form state.
+   *
+   * @return array
+   *   An array with the following key/data pairs:
+   *   - items_count: The number of widgets to display for the field.
+   *   - array_parents: The location of the field's widgets within the $form
+   *     structure. This entry is populated at '#after_build' time.
+   *   - constraint_violations: The array of validation errors reported on the
+   *     field. This entry is populated at form validate time.
+   */
+  public static function getWidgetState(array $parents, $field_name, array &$form_state);
+
+  /**
+   * Stores processing information about the widget in $form_state.
+   *
+   * This method is static so that is can be used in static Form API #callbacks.
+   *
+   * @param array $parents
+   *   The array of #parents where the widget lives in the form.
+   * @param string $field_name
+   *   The field name.
+   * @param array $form_state
+   *   The form state.
+   * @param array $field_state
+   *   The array of data to store. See getWidgetState() for the structure and
+   *   content of the array.
+   */
+  public static function setWidgetState(array $parents, $field_name, array &$form_state, array $field_state);
+
 }
diff --git a/core/lib/Drupal/Core/Field/WidgetInterface.php b/core/lib/Drupal/Core/Field/WidgetInterface.php
index 1edfe45..13d73d2 100644
--- a/core/lib/Drupal/Core/Field/WidgetInterface.php
+++ b/core/lib/Drupal/Core/Field/WidgetInterface.php
@@ -88,8 +88,8 @@ public function settingsSummary();
    *       widgets can simply overlook this property. This identifies the
    *       location where the field values are placed within
    *       $form_state['values'], and is used to access processing information
-   *       for the field through the field_form_get_state() and
-   *       field_form_set_state() functions.
+   *       for the field through the getWidgetState() and setWidgetState()
+   *       methods.
    *   - #title: The sanitized element label for the field instance, ready for
    *     output.
    *   - #description: The sanitized element description for the field instance,
diff --git a/core/modules/field/field.form.inc b/core/modules/field/field.form.inc
deleted file mode 100644
index 6903657..0000000
--- a/core/modules/field/field.form.inc
+++ /dev/null
@@ -1,111 +0,0 @@
-<?php
-
-/**
- * @file
- * Field forms management.
- */
-
-use Drupal\Component\Utility\NestedArray;
-
-/**
- * Callback for usort() within field-multiple-value-form.html.twig.
- *
- * Sorts using ['_weight']['#value']
- */
-function _field_sort_items_value_helper($a, $b) {
-  $a_weight = (is_array($a) && isset($a['_weight']['#value']) ? $a['_weight']['#value'] : 0);
-  $b_weight = (is_array($b) && isset($b['_weight']['#value']) ? $b['_weight']['#value'] : 0);
-  return $a_weight - $b_weight;
-}
-
-/**
- * After-build callback for field elements in a form.
- *
- * This stores the final location of the field within the form structure so that
- * field_default_form_errors() can assign validation errors to the right form
- * element.
- *
- * @param $element
- *   The form element.
- * @param $form_state
- *   An associative array containing the current state of the form.
- *
- * @return
- *   The $element array that was passed in as a parameter.
- *
- * @see field_default_form_errors()
- */
-function field_form_element_after_build($element, &$form_state) {
-  $parents = $element['#field_parents'];
-  $field_name = $element['#field_name'];
-
-  $field_state = field_form_get_state($parents, $field_name, $form_state);
-  $field_state['array_parents'] = $element['#array_parents'];
-  field_form_set_state($parents, $field_name, $form_state, $field_state);
-
-  return $element;
-}
-
-/**
- * Retrieves processing information about a field from $form_state.
- *
- * @param array $parents
- *   The array of #parents where the field lives in the form.
- * @param $field_name
- *   The field name.
- * @param array $form_state
- *   The form state.
- *
- * @return
- *   An array with the following key/data pairs:
- *   - items_count: The number of widgets to display for the field.
- *   - array_parents: The location of the field's widgets within the $form
- *     structure. This entry is populated at '#after_build' time.
- *   - constraint_violations: The array of validation errors reported on the
- *     field. This entry is populated at form validate time.
- *
- * @see field_form_set_state()
- */
-function field_form_get_state(array $parents, $field_name, array &$form_state) {
-  $form_state_parents = _field_form_state_parents($parents, $field_name);
-  return NestedArray::getValue($form_state, $form_state_parents);
-}
-
-/**
- * Stores processing information about a field in $form_state.
- *
- * @param array $parents
- *   The array of #parents where the field lives in the form.
- * @param $field_name
- *   The field name.
- * @param array $form_state
- *   The form state.
- * @param array $field_state
- *   The array of data to store. See field_form_get_state() for the structure
- *   and content of the array.
- *
- * @see field_form_get_state()
- */
-function field_form_set_state(array $parents, $field_name, array &$form_state, array $field_state) {
-  $form_state_parents = _field_form_state_parents($parents, $field_name);
-  NestedArray::setValue($form_state, $form_state_parents, $field_state);
-}
-
-/**
- * Returns the location of processing information within $form_state.
- *
- * @param array $parents
- *   The array of #parents where the field lives in the form.
- * @param $field_name
- *   The field name.
- *
- * @return
- *   The location of processing information within $form_state.
- */
-function _field_form_state_parents(array $parents, $field_name) {
-  // Field processing data is placed at
-  // $form_state['field']['#parents'][...$parents...]['#fields'][$field_name],
-  // to avoid clashes between field names and $parents parts.
-  return array_merge(array('field', '#parents'), $parents, array('#fields', $field_name));
-}
-
diff --git a/core/modules/field/field.module b/core/modules/field/field.module
index 4726101..960aff1 100644
--- a/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -16,7 +16,7 @@
  * mechanism for auto-loading core APIs, so we have to load them on
  * every page request.
  */
-require_once __DIR__ . '/field.form.inc';
+
 require_once __DIR__ . '/field.purge.inc';
 
 /**
diff --git a/core/modules/file/file.field.inc b/core/modules/file/file.field.inc
index 68a0f00..57cd172 100644
--- a/core/modules/file/file.field.inc
+++ b/core/modules/file/file.field.inc
@@ -65,7 +65,7 @@ function template_preprocess_file_widget_multiple(&$variables) {
   foreach (Element::children($element) as $key) {
     $widgets[] = &$element[$key];
   }
-  usort($widgets, '_field_sort_items_value_helper');
+  usort($widgets, '_field_multiple_value_form_sort_helper');
 
   $rows = array();
   foreach ($widgets as $key => &$widget) {
diff --git a/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php b/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
index b9b900f..be49915 100644
--- a/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
+++ b/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
@@ -76,7 +76,7 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
     // Load the items for form rebuilds from the field state as they might not be
     // in $form_state['values'] because of validation limitations. Also, they are
     // only passed in as $items when editing existing entities.
-    $field_state = field_form_get_state($parents, $field_name, $form_state);
+    $field_state = static::getWidgetState($parents, $field_name, $form_state);
     if (isset($field_state['items'])) {
       $items->setValue($field_state['items']);
     }
@@ -528,9 +528,9 @@ public static function submit($form, &$form_state) {
     NestedArray::setValue($form_state['values'], array_slice($button['#parents'], 0, -2), $submitted_values);
 
     // Update items.
-    $field_state = field_form_get_state($parents, $field_name, $form_state);
+    $field_state = static::getWidgetState($parents, $field_name, $form_state);
     $field_state['items'] = $submitted_values;
-    field_form_set_state($parents, $field_name, $form_state, $field_state);
+    static::setWidgetState($parents, $field_name, $form_state, $field_state);
   }
 
 }