diff --git a/core/includes/file.inc b/core/includes/file.inc
index d0ed095..dad8c61 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -191,12 +191,16 @@ function file_get_stream_wrappers($filter = STREAM_WRAPPERS_ALL) {
   $wrappers_storage = &drupal_static(__FUNCTION__);
 
   if (!isset($wrappers_storage)) {
-    $wrappers = \Drupal::moduleHandler()->invokeAll('stream_wrappers');
-    foreach ($wrappers as $scheme => $info) {
-      // Add defaults.
-      $wrappers[$scheme] += array('type' => STREAM_WRAPPERS_NORMAL);
+    $wrappers = array();
+    $container = \Drupal::getContainer();
+    if (is_object($container) && $container->has('module_handler')) {
+      $wrappers = \Drupal::moduleHandler()->invokeAll('stream_wrappers');
+      foreach ($wrappers as $scheme => $info) {
+        // Add defaults.
+        $wrappers[$scheme] += array('type' => STREAM_WRAPPERS_NORMAL);
+      }
+      drupal_alter('stream_wrappers', $wrappers);
     }
-    drupal_alter('stream_wrappers', $wrappers);
     $existing = stream_get_wrappers();
     foreach ($wrappers as $scheme => $info) {
       // We only register classes that implement our interface.
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index b7848de..d90c210 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -88,6 +88,19 @@ protected function setUp() {
     $this->keyValueFactory = new KeyValueMemoryFactory();
 
     parent::setUp();
+
+    // UnitTestBase created a public files directory. Tests based on
+    // DrupalUnitTestBase are entitled to use Drupal's File API, so inject a
+    // 'public' stream wrapper.
+    // @todo Revamp Drupal's stream wrapper API for D8.
+    stream_wrapper_register('public', 'Drupal\Core\StreamWrapper\PublicStream');
+    $wrappers = &drupal_static('file_get_stream_wrappers');
+    $wrappers['public'] = array(
+      'type' => STREAM_WRAPPERS_LOCAL_NORMAL,
+      'class' => 'Drupal\Core\StreamWrapper\PublicStream',
+    );
+    $wrappers[STREAM_WRAPPERS_ALL] = $wrappers;
+
     // Build a minimal, partially mocked environment for unit tests.
     $this->containerBuild(\Drupal::getContainer());
     // Make sure it survives kernel rebuilds.
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 669c615..2af069f 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -963,6 +963,12 @@ protected function prepareEnvironment() {
     // Create and set new configuration directories.
     $this->prepareConfigDirectories();
 
+    // Unregister all Drupal stream wrappers.
+    $wrappers = file_get_stream_wrappers();
+    foreach ($wrappers as $scheme => $info) {
+      stream_wrapper_unregister($scheme);
+    }
+
     // Reset statics before the old container is replaced so that objects with a
     // __destruct() method still have access to it.
     // @todo: Remove once they have been converted to services.