diff --git a/core/modules/config/src/Tests/ConfigEntityTest.php b/core/modules/config/src/Tests/ConfigEntityTest.php
index 7cf16e6..1de0264 100644
--- a/core/modules/config/src/Tests/ConfigEntityTest.php
+++ b/core/modules/config/src/Tests/ConfigEntityTest.php
@@ -319,6 +319,44 @@ function testCRUDUI() {
     $this->drupalPostForm('admin/structure/config_test/manage/0/delete', array(), 'Delete');
     $this->assertFalse(entity_load('config_test', '0'), 'Test entity deleted');
 
+    // Create a configuration entity with a property that uses AJAX to show
+    // extra form elements.
+    $this->drupalGet('admin/structure/config_test/add');
+
+    // Test that the dependent element is not shown initially.
+    $this->assertFieldByName('size');
+    $this->assertNoFieldByName('size_value');
+
+    $id = strtolower($this->randomMachineName());
+    $edit = array(
+      'id' => $id,
+      'label' => $this->randomString(),
+      'size' => 'custom',
+    );
+    $this->drupalPostAjaxForm(NULL, $edit, 'size');
+
+    // Check that the dependent element is shown after selecting a 'size' value.
+    $this->assertFieldByName('size');
+    $this->assertFieldByName('size_value');
+
+    // Test the same scenario but it in a non-js case by using a js-hidden
+    // submit button.
+    $this->drupalGet('admin/structure/config_test/add');
+    $this->assertFieldByName('size');
+    $this->assertNoFieldByName('size_value');
+
+    $this->drupalPostForm(NULL, $edit, 'Change size');
+    $this->assertFieldByName('size');
+    $this->assertFieldByName('size_value');
+
+    // Submit the form with the regular 'Save' button and check that the entity
+    // values are correct.
+    $edit += array('size_value' => 'medium');
+    $this->drupalPostForm(NULL, $edit, 'Save');
+
+    $entity = entity_load('config_test', $id);
+    $this->assertEqual($entity->get('size'), 'custom');
+    $this->assertEqual($entity->get('size_value'), 'medium');
   }
 
 }
diff --git a/core/modules/config/src/Tests/ConfigImportUITest.php b/core/modules/config/src/Tests/ConfigImportUITest.php
index 9bc30e3..4f3aea5 100644
--- a/core/modules/config/src/Tests/ConfigImportUITest.php
+++ b/core/modules/config/src/Tests/ConfigImportUITest.php
@@ -68,6 +68,8 @@ function testImport() {
       'label' => 'New',
       'weight' => 0,
       'style' => '',
+      'size' => '',
+      'size_value' => '',
       'protected_property' => '',
     );
     $staging->write($dynamic_name, $original_dynamic_data);
@@ -373,7 +375,9 @@ function testImportErrorLog() {
       'label' => 'Primary',
       'weight' => 0,
       'style' => NULL,
-      'protected_property' => null,
+      'size' => NULL,
+      'size_value' => NULL,
+      'protected_property' => NULL,
     );
     $staging->write($name_primary, $values_primary);
     $values_secondary = array(
@@ -388,7 +392,9 @@ function testImportErrorLog() {
       'label' => 'Secondary Sync',
       'weight' => 0,
       'style' => NULL,
-      'protected_property' => null,
+      'size' => NULL,
+      'size_value' => NULL,
+      'protected_property' => NULL,
     );
     $staging->write($name_secondary, $values_secondary);
     // Verify that there are configuration differences to import.
diff --git a/core/modules/config/src/Tests/ConfigImporterTest.php b/core/modules/config/src/Tests/ConfigImporterTest.php
index 581b5c4..a6b2312 100644
--- a/core/modules/config/src/Tests/ConfigImporterTest.php
+++ b/core/modules/config/src/Tests/ConfigImporterTest.php
@@ -173,6 +173,8 @@ function testNew() {
       'label' => 'New',
       'weight' => 0,
       'style' => '',
+      'size' => '',
+      'size_value' => '',
       'protected_property' => '',
     );
     $staging->write($dynamic_name, $original_dynamic_data);
diff --git a/core/modules/config/tests/config_test/config/schema/config_test.schema.yml b/core/modules/config/tests/config_test/config/schema/config_test.schema.yml
index 9240d4b..f68acde 100644
--- a/core/modules/config/tests/config_test/config/schema/config_test.schema.yml
+++ b/core/modules/config/tests/config_test/config/schema/config_test.schema.yml
@@ -15,6 +15,12 @@ config_test_dynamic:
     style:
       type: string
       label: 'style'
+    size:
+      type: string
+      label: 'Size'
+    size_value:
+      type: string
+      label: 'Size value'
     protected_property:
       type: string
       label: 'Protected property'
diff --git a/core/modules/config/tests/config_test/src/ConfigTestForm.php b/core/modules/config/tests/config_test/src/ConfigTestForm.php
index e77ddb0..975e27b 100644
--- a/core/modules/config/tests/config_test/src/ConfigTestForm.php
+++ b/core/modules/config/tests/config_test/src/ConfigTestForm.php
@@ -54,6 +54,50 @@ public function form(array $form, FormStateInterface $form_state) {
       $form['style']['#options'] = image_style_options();
     }
 
+    // The main premise of entity forms is that we get to work with an entity
+    // object at all times instead of checking submitted values from the form
+    // state.
+    $size = $entity->get('size');
+
+    $form['size_wrapper'] = array(
+      '#type' => 'container',
+      '#attributes' => array(
+        'id' => 'size-wrapper',
+      ),
+    );
+    $form['size_wrapper']['size'] = array(
+      '#type' => 'select',
+      '#title' => 'Size',
+      '#options' => array(
+        'custom' => 'Custom',
+      ),
+      '#empty_option' => '- None -',
+      '#default_value' => $size,
+      '#ajax' => array(
+        'callback' => '::updateSize',
+        'wrapper' => 'size-wrapper',
+      ),
+    );
+    $form['size_wrapper']['size_submit'] = array(
+      '#type' => 'submit',
+      '#value' => t('Change size'),
+      '#attributes' => array(
+        'class' => array('js-hide'),
+      ),
+      '#submit' => array(array(get_class($this), 'changeSize')),
+    );
+    $form['size_wrapper']['size_value'] = array(
+      '#type' => 'select',
+      '#title' => 'Custom size value',
+      '#options' => array(
+        'small' => 'Small',
+        'medium' => 'Medium',
+        'large' => 'Large',
+      ),
+      '#default_value' => $entity->get('size_value'),
+      '#access' => !empty($size),
+    );
+
     $form['actions'] = array('#type' => 'actions');
     $form['actions']['submit'] = array(
       '#type' => 'submit',
@@ -68,6 +112,20 @@ public function form(array $form, FormStateInterface $form_state) {
   }
 
   /**
+   * Ajax callback for the size selection element.
+   */
+  public static function updateSize(array $form, FormStateInterface $form_state) {
+    return $form['size_wrapper'];
+  }
+
+  /**
+   * Element submit handler for non-js testing.
+   */
+  public static function changeSize(array $form, FormStateInterface $form_state) {
+    $form_state->setRebuild();
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function save(array $form, FormStateInterface $form_state) {