diff -u b/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml b/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml
--- b/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml
+++ b/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml
@@ -16 +16 @@
-              label: 'Is it wonderful?'
\ No newline at end of file
+              label: 'Is it wonderful?'
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
+++ b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
@@ -12,6 +12,7 @@
 use \Countable;
 use \IteratorAggregate;
 use \Traversable;
+use Drupal\Component\Utility\String;
 
 /**
  * Defines a generic configuration element that contains multiple properties.
@@ -114,4 +115,40 @@ public function getIterator() {
     return new ArrayIterator($this->getElements());
   }
 
+  /**
+   * Returns the schema element for the specified key.
+   *
+   * All configuration objects at their root are either of the type
+   * sequence or mapping which both extend ArrayElement. Enable traversal of the
+   * schema by excepting a dot delimited key to access nested values, for
+   * example, 'page.front'.
+   *
+   * This implements the get() method on the follow schema classes:
+   * - \Drupal\Core\Config\Schema\Mapping
+   * - \Drupal\Core\Config\Schema\Sequence
+   * To conform with the respective interfaces:
+   * - \Drupal\Core\TypedData\ComplexDataInterface
+   * - \Drupal\Core\TypedData\ListInterface
+   */
+  public function get($property_name) {
+    $parts = explode('.', $property_name);
+    $root_key = array_shift($parts);
+    $elements = $this->getElements();
+    if (isset($elements[$root_key])) {
+      $element = $elements[$root_key];
+    }
+    else {
+      throw new SchemaIncompleteException(String::format("The configuration property @key doesn't exist.", array('@key' => $property_name)));
+    }
+
+    // If $property_name contained a dot recurse into the keys.
+    foreach ($parts as $key) {
+      if (!is_object($element) || !method_exists($element, 'get')) {
+        throw new SchemaIncompleteException(String::format("The configuration property @key does not exist.", array('@key' => $property_name)));
+      }
+      $element = $element->get($key);
+    }
+    return $element;
+  }
+
 }
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/Schema/Mapping.php
+++ b/core/lib/Drupal/Core/Config/Schema/Mapping.php
@@ -8,7 +8,6 @@
 namespace Drupal\Core\Config\Schema;
 
 use Drupal\Core\TypedData\ComplexDataInterface;
-use Drupal\Component\Utility\String;
 
 /**
  * Defines a mapping configuration element.
@@ -33,33 +32,6 @@ protected function parse() {
   }
 
   /**
-   * Implements Drupal\Core\TypedData\ComplexDataInterface::get().
-   *
-   * Since all configuration objects are mappings the function will except a dot
-   * delimited key to access nested values, for example, 'page.front'.
-   */
-  public function get($property_name) {
-    $parts = explode('.', $property_name);
-    $root_key = array_shift($parts);
-    $elements = $this->getElements();
-    if (isset($elements[$root_key])) {
-      $element = $elements[$root_key];
-    }
-    else {
-      throw new SchemaIncompleteException(String::format("The configuration property @key doesn't exist.", array('@key' => $property_name)));
-    }
-
-    // If $property_name contained a dot recurse into the keys.
-    foreach ($parts as $key) {
-     if (!is_object($element) || !method_exists($element, 'get')) {
-        throw new SchemaIncompleteException(String::format("The configuration property @key does not exist.", array('@key' => $property_name)));
-      }
-      $element = $element->get($key);
-    }
-    return $element;
-  }
-
-  /**
    * Implements Drupal\Core\TypedData\ComplexDataInterface::set().
    */
   public function set($property_name, $value, $notify = TRUE) {
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/Schema/Sequence.php
+++ b/core/lib/Drupal/Core/Config/Schema/Sequence.php
@@ -53,14 +53,6 @@ public function onChange($delta) {
   /**
    * {@inheritdoc}
    */
-  public function get($key) {
-    $elements = $this->getElements();
-    return $elements[$key];
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function first() {
     return $this->get(0);
   }