diff -u b/core/lib/Drupal/Core/Config/Config.php b/core/lib/Drupal/Core/Config/Config.php
--- b/core/lib/Drupal/Core/Config/Config.php
+++ b/core/lib/Drupal/Core/Config/Config.php
@@ -302,24 +302,18 @@
    * Unsets value in this config object.
    *
    * @param string $key
-   *   (optional) The name of the key whose value should be unset. If omitted,
-   *   the entire config object is emptied.
+   *   Name of the key whose value should be unset.
    *
    * @return Drupal\Core\Config\Config
    *   The configuration object.
    */
-  public function clear($key = NULL) {
-    if (!isset($key)) {
-      $this->data = array();
+  public function clear($key) {
+    $parts = explode('.', $key);
+    if (count($parts) == 1) {
+      unset($this->data[$key]);
     }
     else {
-      $parts = explode('.', $key);
-      if (count($parts) == 1) {
-        unset($this->data[$key]);
-      }
-      else {
-        NestedArray::unsetValue($this->data, $parts);
-      }
+      NestedArray::unsetValue($this->data, $parts);
     }
     $this->resetOverriddenData();
     return $this;
reverted:
--- b/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
+++ a/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
@@ -274,10 +274,6 @@
     $this->preSave($entity);
     $this->invokeHook('presave', $entity);
 
-    // Clear out any possibly existing keys in an existing configuration object,
-    // so any potentially stale keys are removed.
-    $config->clear();
-
     // Configuration objects do not have a schema. Extract all key names from
     // class properties.
     $class_info = new \ReflectionClass($entity);
diff -u b/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php
--- b/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigCRUDTest.php
@@ -93,9 +93,9 @@
   }
 
   /**
-   * Tests key injection order.
+   * Tests Drupal\Core\Config\Config::sortByKey().
    */
-  function testDataKeySort() {
+  function testDataKeyOrderPreservation() {
     $config = config('config_test.keysort');
     $config->set('new', 'Value to be replaced');
     $config->set('static', 'static');
@@ -107,13 +107,15 @@
     // Load the configuration data into a new object.
     $new_config = config('config_test.keysort');
+    // Clear the 'new' key that came first.
+    $new_config->clear('new');
     // Add a new 'new' key and save.
     $new_config->set('new', 'Value to be replaced');
     $new_config->save();
 
-    // Verify that the data of both objects is in the identical order.
+    // Verify that the data of objects is in the opposite order.
     // assertIdentical() is the required essence of this test; it performs a
     // strict comparison, which means that keys and values must be identical and
     // their order must be identical.
-    $this->assertIdentical($new_config->get(), $config->get());
+    $this->assertIdentical($new_config->get(), array_reverse($config->get()));
   }
 }
reverted:
--- b/core/modules/config/lib/Drupal/config/Tests/ConfigEntityTest.php
+++ a/core/modules/config/lib/Drupal/config/Tests/ConfigEntityTest.php
@@ -85,19 +85,6 @@
     $this->assertResponse(200);
     $this->assertNoText($label1);
     $this->assertText($label3);
-    $id = $edit['id'];
-
-    // Add an orphan key to the configuration entity's object.
-    $config = config('config_test.dynamic.' . $id);
-    $config->set('foo', 'bar')->save();
-
-    // Re-save the configuration entity.
-    $this->drupalPost('admin/structure/config_test/manage/' . $id, array(), 'Save');
-    $this->assertResponse(200);
-
-    // Verify that the orphan key was destroyed.
-    $config = config('config_test.dynamic.' . $id);
-    $this->assertNull($config->get('foo'));
   }
 
 }