diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 3d81fb2..8bba317 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -326,10 +326,11 @@ public function offsetUnset($offset) {
   /**
    * {@inheritdoc}
    */
-  public function setIfNotExists($key, $value) {
-    if (!array_key_exists($key, $this->formState)) {
-      $this->set($key, $value);
+  public function setIfNotExists($property, $value) {
+    if (!array_key_exists($property, $this->formState)) {
+      $this->set($property, $value);
     }
+    return $this;
   }
 
   /**
@@ -348,6 +349,7 @@ public function &get($property) {
    */
   public function set($property, $value) {
     $this->formState[$property] = $value;
+    return $this;
   }
 
   /**
@@ -417,7 +419,7 @@ public function setErrorByName($name, $message = '') {
       }
     }
 
-    return $this['errors'];
+    return $this;
   }
 
   /**
@@ -425,6 +427,7 @@ public function setErrorByName($name, $message = '') {
    */
   public function setError(&$element, $message = '') {
     $this->setErrorByName(implode('][', $element['#parents']), $message);
+    return $this;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index 7eb07bc..1748fca 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -56,17 +56,16 @@ public function getCacheableArray();
   public function setFormState(array $form_state_additions);
 
   /**
-   * Saves a value for a given key if it does not exist yet.
+   * Sets a value to an arbitrary property if it does not exist yet.
    *
-   * @param string $key
-   *   The key of the data to store.
+   * @param string $property
+   *   The property to use for the value.
    * @param mixed $value
    *   The data to store.
    *
-   * @return bool
-   *   TRUE if the data was set, FALSE if it already existed.
+   * @return $this
    */
-  public function setIfNotExists($key, $value);
+  public function setIfNotExists($property, $value);
 
   /**
    * Sets the redirect URL for the form.
@@ -201,14 +200,19 @@ public static function hasAnyErrors();
    * @param string $message
    *   (optional) The error message to present to the user.
    *
-   * @return mixed
-   *   Return value is for internal use only. To get a list of errors, use
-   *   FormErrorInterface::getErrors() or FormErrorInterface::getError().
+   * @return $this
    */
   public function setErrorByName($name, $message = '');
 
   /**
    * Flags an element as having an error.
+   *
+   * @param array $element
+   *   The form element.
+   * @param string $message
+   *   (optional) The error message to present to the user.
+   *
+   * @return $this
    */
   public function setError(&$element, $message = '');