diff -u b/core/lib/Drupal/Core/Template/AttributeArray.php b/core/lib/Drupal/Core/Template/AttributeArray.php
--- b/core/lib/Drupal/Core/Template/AttributeArray.php
+++ b/core/lib/Drupal/Core/Template/AttributeArray.php
@@ -72,7 +72,8 @@
   /**
    * Adds the argument values by merging them on to the value array.
    *
-   * @return $this
+   * @return \Drupal\Core\Template\AttributeArray
+   *   The attribute array with the arguments added.
    */
   public function add() {
     $args = func_get_args();
@@ -91,7 +92,8 @@
   /**
    * Removes the argument values from the value array.
    *
-   * @return $this
+   * @return \Drupal\Core\Template\AttributeArray
+   *   The attribute array with the arguments removed.
    */
   public function remove() {
     $args = func_get_args();
interdiff impossible; taking evasive action
reverted:
--- b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
@@ -66,56 +66,6 @@
     $this->assertFalse(isset($attribute['class']));
   }
 
-
-  /**
-   * Tests adding class attributes with the AttributeArray helper method.
-   */
-  public function testAddClasses() {
-    $attribute = new Attribute(array('class' => array('example-class')));
-
-    // Add one class.
-    $attribute['class']->add('aa');
-    $this->assertEquals(new AttributeArray('class', array('example-class', 'aa')), $attribute['class']);
-
-    // Add multiple classes.
-    $attribute['class']->add('xx', 'yy');
-    $this->assertEquals(new AttributeArray('class', array('example-class', 'aa', 'xx', 'yy')), $attribute['class']);
-
-    // Add an array of classes.
-    $attribute['class']->add(array('red', 'green', 'blue'));
-    $this->assertEquals(new AttributeArray('class', array('example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue')), $attribute['class']);
-
-  }
-
-  /**
-   * Tests removing class attributes with the AttributeArray helper method.
-   */
-  public function testRemoveClasses() {
-    $attribute = new Attribute(array('class' => array('example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue')));
-
-    // Remove one class.
-    $attribute['class']->remove('example-class');
-    $this->assertFalse(in_array('example-class', $attribute['class']->value()));
-
-    // Remove multiple classes.
-    $attribute['class']->remove('xx', 'yy');
-    $this->assertFalse(in_array(array('xx', 'yy'), $attribute['class']->value()));
-
-    // Remove an array of classes.
-    $attribute['class']->remove(array('red', 'green', 'blue'));
-    $this->assertFalse(in_array(array('red', 'green', 'blue'), $attribute['class']->value()));
-  }
-
-  /**
-   * Tests removing class attributes with the AttributeArray helper method.
-   */
-  public function testChainAddRemoveClasses() {
-    $attribute = new Attribute(array('class' => array('example-class', 'red', 'green', 'blue')));
-    unset($attribute['class']);
-    $this->assertFalse(isset($attribute['class']));
-  }
-
-
   /**
    * Tests iterating on the values of the attribute.
    */
unchanged:
--- a/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/AttributeTest.php
@@ -57,6 +57,72 @@ public function testRemove() {
     $this->assertFalse(isset($attribute['class']));
   }
 
+
+  /**
+   * Tests adding class attributes with the AttributeArray helper method.
+   */
+  public function testAddClasses() {
+    $attribute = new Attribute(array('class' => array('example-class')));
+
+    // Add one class.
+    $attribute['class']->add('aa');
+    $this->assertEquals(new AttributeArray('class', array('example-class', 'aa')), $attribute['class']);
+
+    // Add multiple classes.
+    $attribute['class']->add('xx', 'yy');
+    $this->assertEquals(
+      new AttributeArray(
+        'class',
+        array('example-class', 'aa', 'xx', 'yy')),
+      $attribute['class']
+    );
+
+    // Add an array of classes.
+    $attribute['class']->add(array('red', 'green', 'blue'));
+    $this->assertEquals(
+      new AttributeArray(
+        'class',
+        array('example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue')
+      ),
+      $attribute['class']
+    );
+
+  }
+
+  /**
+   * Tests removing class attributes with the AttributeArray helper method.
+   */
+  public function testRemoveClasses() {
+    $attribute = new Attribute(
+      array(
+        'class' => array('example-class', 'aa', 'xx', 'yy', 'red', 'green', 'blue'),
+      )
+    );
+
+    // Remove one class.
+    $attribute['class']->remove('example-class');
+    $this->assertFalse(in_array('example-class', $attribute['class']->value()));
+
+    // Remove multiple classes.
+    $attribute['class']->remove('xx', 'yy');
+    $this->assertFalse(in_array(array('xx', 'yy'), $attribute['class']->value()));
+
+    // Remove an array of classes.
+    $attribute['class']->remove(array('red', 'green', 'blue'));
+    $this->assertFalse(in_array(array('red', 'green', 'blue'), $attribute['class']->value()));
+  }
+
+  /**
+   * Tests removing class attributes with the AttributeArray helper method.
+   */
+  public function testChainAddRemoveClasses() {
+    $attribute = new Attribute(
+      array('class' => array('example-class', 'red', 'green', 'blue'))
+    );
+    unset($attribute['class']);
+    $this->assertFalse(isset($attribute['class']));
+  }
+
   /**
    * Tests iterating on the values of the attribute.
    */