.../Unit/Normalizer/ConfigEntityNormalizerTest.php | 57 ++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
index 2f99e2d..0a27c80 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/ConfigEntityNormalizerTest.php
@@ -2,6 +2,9 @@
 
 namespace Drupal\Tests\serialization\Unit\Normalizer;
 
+use Drupal\Core\Config\Entity\ConfigEntityTypeInterface;
+use Drupal\Core\Entity\EntityManagerInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\serialization\Normalizer\ConfigEntityNormalizer;
 use Drupal\Tests\UnitTestCase;
 
@@ -36,4 +39,58 @@ public function testNormalize() {
     $this->assertSame(['test' => 'test'], $normalizer->normalize($config_entity));
   }
 
+  /**
+   * @covers ::denormalize
+   */
+  public function testDenormalize() {
+    $test_value = $this->randomMachineName();
+    $data = [
+      'test' => $test_value,
+      '_core' => [
+        'default_config_hash' => $this->randomMachineName(),
+        $this->randomMachineName() => 'some random key',
+      ],
+    ];
+
+    $expected_storage_data = [
+      'test' => $test_value,
+    ];
+
+    // Mock of the entity storage, to test our expectation that the '_core' key
+    // never makes it to that point, thanks to the denormalizer omitting it.
+    $entity_storage = $this->prophesize(EntityStorageInterface::class);
+    $entity_storage->create($expected_storage_data)
+      ->shouldBeCalled()
+      ->will(function ($args) {
+        $entity = new \stdClass();
+        $entity->received_data = $args[0];
+        return $entity;
+      });
+
+    // Stubs for the denormalizer going from entity manager to entity storage.
+    $entity_type_id = $this->randomMachineName();
+    $entity_type_class = $this->randomMachineName();
+    $entity_manager = $this->prophesize(EntityManagerInterface::class);
+    $entity_manager->getEntityTypeFromClass($entity_type_class)
+      ->willReturn($entity_type_id);
+    $entity_manager->getDefinition($entity_type_id, FALSE)
+      ->willReturn($this->prophesize(ConfigEntityTypeInterface::class)->reveal());
+    $entity_manager->getStorage($entity_type_id)
+      ->willReturn($entity_storage->reveal());
+    $normalizer = new ConfigEntityNormalizer($entity_manager->reveal());
+
+    // Verify the denormalizer still works correctly: the mock above creates an
+    // artificial entity object containing exactly the data it received. It also
+    // should still set _restSubmittedFields correctly.
+    $expected_denormalization = (object) [
+      '_restSubmittedFields' => [
+        'test',
+      ],
+      'received_data' => [
+        'test' => $test_value,
+      ],
+    ];
+    $this->assertEquals($expected_denormalization, $normalizer->denormalize($data, $entity_type_class, 'json'));
+  }
+
 }