Update to Drupal 8.1.1. For more information, see https://www.drupal.org/node/2718713

core/modules/rest/tests/src/Kernel/RequestHandlerTest.php

@@ -126,9 +126,11 @@ class RequestHandlerTest extends KernelTestBase {
 
 }
 
+/**
+ * Stub class where we can prophesize methods.
+ */
 class StubRequestHandlerResourcePlugin extends ResourceBase {
 
-  /** stub methods so they can be prophesied. */
   function get() {}
   function patch() {}
 }

core/modules/rest/tests/src/Kernel/RestLinkManagerTest.php

@@ -1,8 +1,9 @@
 <?php
 
-namespace Drupal\rest\Tests;
+namespace Drupal\Tests\rest\Kernel;
+
 use Drupal\Core\Url;
-use Drupal\simpletest\KernelTestBase;
+use Drupal\KernelTests\KernelTestBase;
 
 /**
  * Tests that REST type and relation link managers work as expected