diff -u b/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php b/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Serialization/EntitySerializationTest.php
@@ -10,7 +10,6 @@
 use Drupal\simpletest\WebTestBase;
 use Symfony\Component\Serializer\Serializer;
 use Drupal\Core\Serialization\JsonEncoder;
-use Symfony\Component\Serializer\Encoder\XmlEncoder as BaseXmlEncoder;
 use Drupal\Core\Serialization\ComplexDataNormalizer;
 use Drupal\Core\Serialization\TypedDataNormalizer;
 
@@ -130,12 +129,33 @@
     $actual = $serializer->serialize($normalized, 'ajax');
     $this->assertIdentical($actual, $expected, 'A normalized array serializes to JSON when "ajax" is requested');
-    // Test 'xml'. The expected output should match that of Symfony's XmlEncoder.
-    $expected ='<?xml version="1.0"?>
-<response><id><value>1</value></id><revision_id><value>1</value></revision_id><uuid><value>' . $this->entity->uuid() . '</value></uuid><langcode><value>und</value></langcode><default_langcode><value/></default_langcode><name><value>' . $this->values['name'] . '</value></name><user_id><value>1</value></user_id><field_test_text><value>' . $this->values['field_test_text']['value'] . '</value><format>full_html</format></field_test_text></response>
-';
+
+    // Generate the expected xml in a way that allows changes to entity property
+    // order.
+    $expected = array(
+      'id' => '<id><value>' . $this->entity->id() . '</value></id>',
+      'revision_id' => '<revision_id><value>' . $this->entity->getRevisionId() . '</value></revision_id>',
+      'uuid' => '<uuid><value>' . $this->entity->uuid() . '</value></uuid>',
+      'langcode' => '<langcode><value>' . LANGUAGE_NOT_SPECIFIED . '</value></langcode>',
+      'default_langcode' => '<default_langcode><value/></default_langcode>',
+      'name' => '<name><value>' . $this->values['name'] . '</value></name>',
+      'user_id' => '<user_id><value>' . $this->values['user_id'] . '</value></user_id>',
+      'field_test_text' => '<field_test_text><value>' . $this->values['field_test_text']['value'] . '</value><format>' . $this->values['field_test_text']['format'] . '</format></field_test_text>',
+    );
+    // Sort it in the same order as normalised.
+    $expected = array_merge(array_flip(array_keys($normalized)), $expected);
+    // Add header and footer.
+    array_unshift($expected, '<?xml version="1.0"?>' . PHP_EOL . '<response>');
+    $expected[] = '</response>' . PHP_EOL;
+    // Reduced the array to a string.
+    $expected = array_reduce($expected, function($result, $item) {
+      $result .= $item;
+      return $result;
+    });
+
+    // Test 'xml'. The output should match that of Symfony's XmlEncoder.
     $actual = $serializer->serialize($this->entity, 'xml');
-    $this->assertIdentical($actual, $expected, 'Entity serializes to XML when "xml" is requested.');
+    $this->assertIdentical($actual, $expected);
     $actual = $serializer->serialize($normalized, 'xml');
-    $this->assertIdentical($actual, $expected, 'A normalized array serializes to XML when "xml" is requested');
+    $this->assertIdentical($actual, $expected);
   }
 }