diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index 2144065..8a6f22b 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -12,7 +12,7 @@
 /**
  * Provides an interface for form building and processing.
  */
-interface FormBuilderInterface {
+interface FormBuilderInterface extends FormErrorInterface {
 
   /**
    * Determines the form ID.
@@ -509,145 +509,6 @@ public function redirectForm($form_state);
   public function executeHandlers($type, &$form, &$form_state);
 
   /**
-   * Files an error against a form element.
-   *
-   * When a validation error is detected, the validator calls form_set_error()
-   * to indicate which element needs to be changed and provide an error message.
-   * This causes the Form API to not execute the form submit handlers, and
-   * instead to re-display the form to the user with the corresponding elements
-   * rendered with an 'error' CSS class (shown as red by default).
-   *
-   * The standard form_set_error() behavior can be changed if a button provides
-   * the #limit_validation_errors property. Multistep forms not wanting to
-   * validate the whole form can set #limit_validation_errors on buttons to
-   * limit validation errors to only certain elements. For example, pressing the
-   * "Previous" button in a multistep form should not fire validation errors
-   * just because the current step has invalid values. If
-   * #limit_validation_errors is set on a clicked button, the button must also
-   * define a #submit property (may be set to an empty array). Any #submit
-   * handlers will be executed even if there is invalid input, so extreme care
-   * should be taken with respect to any actions taken by them. This is
-   * typically not a problem with buttons like "Previous" or "Add more" that do
-   * not invoke persistent storage of the submitted form values. Do not use the
-   * #limit_validation_errors property on buttons that trigger saving of form
-   * values to the database.
-   *
-   * The #limit_validation_errors property is a list of "sections" within
-   * $form_state['values'] that must contain valid values. Each "section" is an
-   * array with the ordered set of keys needed to reach that part of
-   * $form_state['values'] (i.e., the #parents property of the element).
-   *
-   * Example 1: Allow the "Previous" button to function, regardless of whether
-   * any user input is valid.
-   *
-   * @code
-   *   $form['actions']['previous'] = array(
-   *     '#type' => 'submit',
-   *     '#value' => t('Previous'),
-   *     '#limit_validation_errors' => array(),       // No validation.
-   *     '#submit' => array('some_submit_function'),  // #submit required.
-   *   );
-   * @endcode
-   *
-   * Example 2: Require some, but not all, user input to be valid to process the
-   * submission of a "Previous" button.
-   *
-   * @code
-   *   $form['actions']['previous'] = array(
-   *     '#type' => 'submit',
-   *     '#value' => t('Previous'),
-   *     '#limit_validation_errors' => array(
-   *       array('step1'),      // Validate $form_state['values']['step1'].
-   *       array('foo', 'bar'), // Validate $form_state['values']['foo']['bar'].
-   *     ),
-   *     '#submit' => array('some_submit_function'), // #submit required.
-   *   );
-   * @endcode
-   *
-   * This will require $form_state['values']['step1'] and everything within it
-   * (for example, $form_state['values']['step1']['choice']) to be valid, so
-   * calls to form_set_error('step1', $form_state, $message) or
-   * form_set_error('step1][choice', $form_state, $message) will prevent the
-   * submit handlers from running, and result in the error message being
-   * displayed to the user. However, calls to
-   * form_set_error('step2', $form_state, $message) and
-   * form_set_error('step2][groupX][choiceY', $form_state, $message) will be
-   * suppressed, resulting in the message not being displayed to the user, and
-   * the submitdoCheckErrors handlers will run despite $form_state['values']['step2'] and
-   * $form_state['values']['step2']['groupX']['choiceY'] containing invalid
-   * values. Errors for an invalid $form_state['values']['foo'] will be
-   * suppressed, but errors flagging invalid values for
-   * $form_state['values']['foo']['bar'] and everything within it will be
-   * flagged and submission prevented.
-   *
-   * Partial form validation is implemented by suppressing errors rather than by
-   * skipping the input processing and validation steps entirely, because some
-   * forms have button-level submit handlers that call Drupal API functions that
-   * assume that certain data exists within $form_state['values'], and while not
-   * doing anything with that data that requires it to be valid, PHP errors
-   * would be triggered if the input processing and validation steps were fully
-   * skipped.
-   *
-   * @param $name
-   *   The name of the form element. If the #parents property of your form
-   *   element is array('foo', 'bar', 'baz') then you may set an error on 'foo'
-   *   or 'foo][bar][baz'. Setting an error on 'foo' sets an error for every
-   *   element where the #parents array starts with 'foo'.
-   * @param array $form_state
-   *   An associative array containing the current state of the form.
-   * @param $message
-   *   The error message to present to the user.
-   *
-   * @return mixed
-   *   Return value is for internal use only. To get a list of errors, use
-   *   form_get_errors() or form_get_error().
-   *
-   * @see http://drupal.org/node/370537
-   * @see http://drupal.org/node/763376
-   */
-  public function setErrorByName($name, array &$form_state, $message = '');
-
-  /**
-   * Clears all errors against all form elements made by form_set_error().
-   *
-   * @param array $form_state
-   *   An associative array containing the current state of the form.
-   */
-  public function clearErrors(array &$form_state);
-
-  /**
-   * Returns an associative array of all errors.
-   *
-   * @param array $form_state
-   *   An associative array containing the current state of the form.
-   *
-   * @return array
-   *   An array of all errors, keyed by the name of the form element.
-   */
-  public function getErrors(array $form_state);
-
-  /**
-   * Returns the error message filed against the given form element.
-   *
-   * Form errors higher up in the form structure override deeper errors as well
-   * as errors on the element itself.
-   *
-   * @param array $element
-   *   The form element to check for errors.
-   * @param array $form_state
-   *   An associative array containing the current state of the form.
-   *
-   * @return string|null
-   *   Either the error message for this element or NULL if there are no errors.
-   */
-  public function getError($element, array &$form_state);
-
-  /**
-   * Flags an element as having an error.
-   */
-  public function setError(&$element, array &$form_state, $message = '');
-
-  /**
    * Builds and processes all elements in the structured form array.
    *
    * Adds any required properties to each element, maps the incoming input data
@@ -781,14 +642,4 @@ public function setValue($element, $value, &$form_state);
    */
   public function setRequest(Request $request);
 
-  /**
-   * Returns if there have been any errors during build.
-   *
-   * This will include any forms built during this request.
-   *
-   * @return bool
-   *   Whether there have been any errors.
-   */
-  public function getAnyErrors();
-
 }
diff --git a/core/lib/Drupal/Core/Form/FormErrorInterface.php b/core/lib/Drupal/Core/Form/FormErrorInterface.php
new file mode 100644
index 0000000..c642ed3
--- /dev/null
+++ b/core/lib/Drupal/Core/Form/FormErrorInterface.php
@@ -0,0 +1,164 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Form\FormErrorInterface.
+ */
+
+namespace Drupal\Core\Form;
+
+/**
+ * Provides an interface for form error handling.
+ */
+interface FormErrorInterface {
+
+  /**
+   * Files an error against a form element.
+   *
+   * When a validation error is detected, the validator calls form_set_error()
+   * to indicate which element needs to be changed and provide an error message.
+   * This causes the Form API to not execute the form submit handlers, and
+   * instead to re-display the form to the user with the corresponding elements
+   * rendered with an 'error' CSS class (shown as red by default).
+   *
+   * The standard form_set_error() behavior can be changed if a button provides
+   * the #limit_validation_errors property. Multistep forms not wanting to
+   * validate the whole form can set #limit_validation_errors on buttons to
+   * limit validation errors to only certain elements. For example, pressing the
+   * "Previous" button in a multistep form should not fire validation errors
+   * just because the current step has invalid values. If
+   * #limit_validation_errors is set on a clicked button, the button must also
+   * define a #submit property (may be set to an empty array). Any #submit
+   * handlers will be executed even if there is invalid input, so extreme care
+   * should be taken with respect to any actions taken by them. This is
+   * typically not a problem with buttons like "Previous" or "Add more" that do
+   * not invoke persistent storage of the submitted form values. Do not use the
+   * #limit_validation_errors property on buttons that trigger saving of form
+   * values to the database.
+   *
+   * The #limit_validation_errors property is a list of "sections" within
+   * $form_state['values'] that must contain valid values. Each "section" is an
+   * array with the ordered set of keys needed to reach that part of
+   * $form_state['values'] (i.e., the #parents property of the element).
+   *
+   * Example 1: Allow the "Previous" button to function, regardless of whether
+   * any user input is valid.
+   *
+   * @code
+   *   $form['actions']['previous'] = array(
+   *     '#type' => 'submit',
+   *     '#value' => t('Previous'),
+   *     '#limit_validation_errors' => array(),       // No validation.
+   *     '#submit' => array('some_submit_function'),  // #submit required.
+   *   );
+   * @endcode
+   *
+   * Example 2: Require some, but not all, user input to be valid to process the
+   * submission of a "Previous" button.
+   *
+   * @code
+   *   $form['actions']['previous'] = array(
+   *     '#type' => 'submit',
+   *     '#value' => t('Previous'),
+   *     '#limit_validation_errors' => array(
+   *       array('step1'),      // Validate $form_state['values']['step1'].
+   *       array('foo', 'bar'), // Validate $form_state['values']['foo']['bar'].
+   *     ),
+   *     '#submit' => array('some_submit_function'), // #submit required.
+   *   );
+   * @endcode
+   *
+   * This will require $form_state['values']['step1'] and everything within it
+   * (for example, $form_state['values']['step1']['choice']) to be valid, so
+   * calls to form_set_error('step1', $form_state, $message) or
+   * form_set_error('step1][choice', $form_state, $message) will prevent the
+   * submit handlers from running, and result in the error message being
+   * displayed to the user. However, calls to
+   * form_set_error('step2', $form_state, $message) and
+   * form_set_error('step2][groupX][choiceY', $form_state, $message) will be
+   * suppressed, resulting in the message not being displayed to the user, and
+   * the submitdoCheckErrors handlers will run despite $form_state['values']['step2'] and
+   * $form_state['values']['step2']['groupX']['choiceY'] containing invalid
+   * values. Errors for an invalid $form_state['values']['foo'] will be
+   * suppressed, but errors flagging invalid values for
+   * $form_state['values']['foo']['bar'] and everything within it will be
+   * flagged and submission prevented.
+   *
+   * Partial form validation is implemented by suppressing errors rather than by
+   * skipping the input processing and validation steps entirely, because some
+   * forms have button-level submit handlers that call Drupal API functions that
+   * assume that certain data exists within $form_state['values'], and while not
+   * doing anything with that data that requires it to be valid, PHP errors
+   * would be triggered if the input processing and validation steps were fully
+   * skipped.
+   *
+   * @param $name
+   *   The name of the form element. If the #parents property of your form
+   *   element is array('foo', 'bar', 'baz') then you may set an error on 'foo'
+   *   or 'foo][bar][baz'. Setting an error on 'foo' sets an error for every
+   *   element where the #parents array starts with 'foo'.
+   * @param array $form_state
+   *   An associative array containing the current state of the form.
+   * @param $message
+   *   The error message to present to the user.
+   *
+   * @return mixed
+   *   Return value is for internal use only. To get a list of errors, use
+   *   form_get_errors() or form_get_error().
+   *
+   * @see http://drupal.org/node/370537
+   * @see http://drupal.org/node/763376
+   */
+  public function setErrorByName($name, array &$form_state, $message = '');
+
+  /**
+   * Clears all errors against all form elements made by form_set_error().
+   *
+   * @param array $form_state
+   *   An associative array containing the current state of the form.
+   */
+  public function clearErrors(array &$form_state);
+
+  /**
+   * Returns an associative array of all errors.
+   *
+   * @param array $form_state
+   *   An associative array containing the current state of the form.
+   *
+   * @return array
+   *   An array of all errors, keyed by the name of the form element.
+   */
+  public function getErrors(array $form_state);
+
+  /**
+   * Returns the error message filed against the given form element.
+   *
+   * Form errors higher up in the form structure override deeper errors as well
+   * as errors on the element itself.
+   *
+   * @param array $element
+   *   The form element to check for errors.
+   * @param array $form_state
+   *   An associative array containing the current state of the form.
+   *
+   * @return string|null
+   *   Either the error message for this element or NULL if there are no errors.
+   */
+  public function getError($element, array &$form_state);
+
+  /**
+   * Flags an element as having an error.
+   */
+  public function setError(&$element, array &$form_state, $message = '');
+
+  /**
+   * Returns if there have been any errors during build.
+   *
+   * This will include any forms built during this request.
+   *
+   * @return bool
+   *   Whether there have been any errors.
+   */
+  public function getAnyErrors();
+
+}