diff --git a/core/includes/file.inc b/core/includes/file.inc
index 545fca3..c487b81 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..89d69b1 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -64,6 +64,13 @@
   protected $keyValueFactory;
 
   /**
+   * A list of stream wrappers that have been registered by this test.
+   *
+   * @var array
+   */
+  private $stream_wrappers = array();
+
+  /**
    * Overrides \Drupal\simpletest\UnitTestBase::__construct().
    */
   function __construct($test_id = NULL) {
@@ -88,6 +95,12 @@ 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.
+    $this->registerStreamWrapper('public', 'Drupal\Core\StreamWrapper\PublicStream');
+
     // Build a minimal, partially mocked environment for unit tests.
     $this->containerBuild(\Drupal::getContainer());
     // Make sure it survives kernel rebuilds.
@@ -134,6 +147,9 @@ protected function setUp() {
 
   protected function tearDown() {
     $this->kernel->shutdown();
+    foreach ($this->stream_wrappers as $scheme) {
+      $this->unregisterStreamWrapper($scheme);
+    }
     parent::tearDown();
   }
 
@@ -334,4 +350,45 @@ protected function disableModules(array $modules) {
     )));
   }
 
+  /**
+   * Registers a stream wrapper for this test.
+   *
+   * @param string $scheme
+   *   The scheme to register.
+   * @param string $class
+   *   The fully qualified class name to register.
+   * @param int $type
+   *   The Drupal Stream Wrapper API type. Defaults to
+   *   STREAM_WRAPPERS_LOCAL_NORMAL.
+   */
+  protected function registerStreamWrapper($scheme, $class, $type = STREAM_WRAPPERS_LOCAL_NORMAL) {
+    if (isset($this->stream_wrappers[$scheme])) {
+      $this->unregisterStreamWrapper($scheme);
+    }
+    $this->stream_wrappers[$scheme] = $scheme;
+    stream_wrapper_register($scheme, $class);
+    // @todo Revamp Drupal's stream wrapper API for D8.
+    $wrappers = &drupal_static('file_get_stream_wrappers');
+    $wrappers[$scheme] = array(
+      'type' => $type,
+      'class' => $class,
+    );
+    $wrappers[STREAM_WRAPPERS_ALL] = $wrappers;
+  }
+
+  /**
+   * Unregisters a stream wrapper previously registered by this test.
+   *
+   * @param string $scheme
+   *   The scheme to unregister.
+   */
+  protected function unregisterStreamWrapper($scheme) {
+    stream_wrapper_unregister($scheme);
+    unset($this->stream_wrappers[$scheme]);
+    // @todo Revamp Drupal's stream wrapper API for D8.
+    $wrappers = &drupal_static('file_get_stream_wrappers');
+    unset($wrappers[$scheme]);
+    unset($wrappers[STREAM_WRAPPERS_ALL][$scheme]);
+  }
+
 }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 669c615..a859d93 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.
@@ -1106,13 +1112,6 @@ protected function tearDown() {
       }
     }
 
-    // In case a fatal error occurred that was not in the test process read the
-    // log to pick up any fatal errors.
-    simpletest_log_read($this->testId, $this->databasePrefix, get_class($this), TRUE);
-
-    // Delete temporary files directory.
-    file_unmanaged_delete_recursive($this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10), array($this, 'filePreDeleteCallback'));
-
     // Restore original database connection.
     Database::removeConnection('default');
     Database::renameConnection('simpletest_original_default', 'default');
@@ -1139,6 +1138,17 @@ protected function tearDown() {
     // Restore original statics and globals.
     \Drupal::setContainer($this->originalContainer);
     $GLOBALS['config_directories'] = $this->originalConfigDirectories;
+
+    // Re-initialize original stream wrappers.
+    file_get_stream_wrappers();
+
+    // In case a fatal error occurred that was not in the test process read the
+    // log to pick up any fatal errors.
+    simpletest_log_read($this->testId, $this->databasePrefix, get_class($this), TRUE);
+
+    // Delete temporary files directory.
+    file_unmanaged_delete_recursive($this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10), array($this, 'filePreDeleteCallback'));
+
     if (isset($this->originalPrefix)) {
       drupal_valid_test_ua($this->originalPrefix);
     }
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/ConfigTest.php b/core/modules/system/lib/Drupal/system/Tests/File/ConfigTest.php
index ce48dfd..a38322b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/ConfigTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/ConfigTest.php
@@ -7,10 +7,12 @@
 
 namespace Drupal\system\Tests\File;
 
+use Drupal\simpletest\WebTestBase;
+
 /**
  * File system configuration related tests.
  */
-class ConfigTest extends FileTestBase {
+class ConfigTest extends WebTestBase {
 
   public static function getInfo() {
     return array(
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/FileTestBase.php b/core/modules/system/lib/Drupal/system/Tests/File/FileTestBase.php
index 540c19a..cc45e7f 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/FileTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/FileTestBase.php
@@ -8,22 +8,30 @@
 namespace Drupal\system\Tests\File;
 
 use Drupal\file\FileInterface;
-use Drupal\simpletest\WebTestBase;
+use Drupal\simpletest\DrupalUnitTestBase;
 
 /**
  * Base class for file tests that adds some additional file specific
  * assertions and helper functions.
  */
-abstract class FileTestBase extends WebTestBase {
+abstract class FileTestBase extends DrupalUnitTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('system');
 
   function setUp() {
     parent::setUp();
-    // Make sure that custom stream wrappers are registered.
-    // @todo This has the potential to be a major bug deeply buried in File API;
-    //   file_unmanaged_*() API functions and test functions are invoking native
-    //   PHP functions directly, whereas Drupal's custom stream wrappers are not
-    //   registered yet.
-    file_get_stream_wrappers();
+    $this->installConfig(array('system'));
+    $this->registerStreamWrapper('private', 'Drupal\Core\StreamWrapper\PrivateStream');
+    $this->registerStreamWrapper('temporary', 'Drupal\Core\StreamWrapper\TemporaryStream');
+
+    if (isset($this->scheme)) {
+      $this->registerStreamWrapper($this->scheme, $this->classname);
+    }
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php b/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php
index 5ee981d..e9c388d 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/MimeTypeTest.php
@@ -7,12 +7,10 @@
 
 namespace Drupal\system\Tests\File;
 
-use Drupal\simpletest\WebTestBase;
-
 /**
  * Tests for file_get_mimetype().
  */
-class MimeTypeTest extends WebTestBase {
+class MimeTypeTest extends FileTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php b/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php
index ef84796..779ba12 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php
@@ -12,13 +12,6 @@
  */
 class ReadOnlyStreamWrapperTest extends FileTestBase {
 
-  /**
-   * Modules to enable.
-   *
-   * @var array
-   */
-  public static $modules = array('file_test');
-
   protected $scheme = 'dummy-readonly';
   protected $classname = 'Drupal\file_test\DummyReadOnlyStreamWrapper';
 
@@ -30,16 +23,6 @@ public static function getInfo() {
     );
   }
 
-  function setUp() {
-    parent::setUp();
-    drupal_static_reset('file_get_stream_wrappers');
-  }
-
-  function tearDown() {
-    parent::tearDown();
-    stream_wrapper_unregister($this->scheme);
-  }
-
   /**
    * Test write functionality of the read-only stream wrapper.
    */
diff --git a/core/modules/system/lib/Drupal/system/Tests/File/StreamWrapperTest.php b/core/modules/system/lib/Drupal/system/Tests/File/StreamWrapperTest.php
index 16bc592..e095ed4 100644
--- a/core/modules/system/lib/Drupal/system/Tests/File/StreamWrapperTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/File/StreamWrapperTest.php
@@ -8,12 +8,11 @@
 namespace Drupal\system\Tests\File;
 
 use Drupal\Core\StreamWrapper\PublicStream;
-use Drupal\simpletest\WebTestBase;
 
 /**
  * Tests stream wrapper functions.
  */
-class StreamWrapperTest extends WebTestBase {
+class StreamWrapperTest extends FileTestBase {
 
   /**
    * Modules to enable.
@@ -33,16 +32,6 @@ public static function getInfo() {
     );
   }
 
-  function setUp() {
-    parent::setUp();
-    drupal_static_reset('file_get_stream_wrappers');
-  }
-
-  function tearDown() {
-    parent::tearDown();
-    stream_wrapper_unregister($this->scheme);
-  }
-
   /**
    * Test the getClassName() function.
    */