diff --git a/core/core.services.yml b/core/core.services.yml
index cab2b0a..29a734a 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -191,7 +191,7 @@ services:
     arguments: [default]
   file_system:
     class: Drupal\Core\File\FileSystem
-    arguments: ['@settings', '@logger.channel.file']
+    arguments: ['@stream_wrapper_manager', '@settings', '@logger.channel.file']
   form_builder:
     class: Drupal\Core\Form\FormBuilder
     arguments: ['@form_validator', '@form_submitter', '@form_cache', '@module_handler', '@event_dispatcher', '@request_stack', '@class_resolver', '@theme.manager', '@?csrf_token']
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 8ad0fc0..5aa79b8 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -340,16 +340,17 @@ function install_begin_request($class_loader, &$install_state) {
   $container
     ->register('string_translation', 'Drupal\Core\StringTranslation\TranslationManager')
     ->addArgument(new Reference('language_manager'));
-  $container
-    ->register('file_system', 'Drupal\Core\File\FileSystem')
-    ->addArgument(Settings::getInstance())
-    ->addArgument((new LoggerChannelFactory())->get('file'));
 
   // Register the stream wrapper manager.
   $container
     ->register('stream_wrapper_manager', 'Drupal\Core\StreamWrapper\StreamWrapperManager')
     ->addArgument(new Reference('module_handler'))
     ->addMethodCall('setContainer', array(new Reference('service_container')));
+  $container
+    ->register('file_system', 'Drupal\Core\File\FileSystem')
+    ->addArgument(new Reference('stream_wrapper_manager'))
+    ->addArgument(Settings::getInstance())
+    ->addArgument((new LoggerChannelFactory())->get('file'));
 
   \Drupal::setContainer($container);
 
diff --git a/core/lib/Drupal/Core/File/FileSystem.php b/core/lib/Drupal/Core/File/FileSystem.php
index 75e9b57..aedfdf6 100644
--- a/core/lib/Drupal/Core/File/FileSystem.php
+++ b/core/lib/Drupal/Core/File/FileSystem.php
@@ -2,18 +2,19 @@
 
 /**
  * @file
- * Contains \Drupal\Core\File\File.
+ * Contains \Drupal\Core\File\FileSystem.
  */
 
 namespace Drupal\Core\File;
 
 use Drupal\Core\Site\Settings;
+use Drupal\Core\StreamWrapper\StreamWrapperManagerInterface;
 use Psr\Log\LoggerInterface;
 
 /**
  * Provides helpers to operate on files and stream wrappers.
  */
-class FileSystem {
+class FileSystem implements FileSystemInterface {
 
   /**
    * Default mode for new directories. See self::chmod().
@@ -40,37 +41,30 @@ class FileSystem {
   protected $logger;
 
   /**
+   * The stream wrapper manager.
+   *
+   * @var \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface
+   */
+  protected $streamWrapperManager;
+
+  /**
    * Constructs a new FileSystem.
    *
+   * @param \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface $stream_wrapper_manager
+   *   The stream wrapper manager.
    * @param \Drupal\Core\Site\Settings $settings
    *   The site settings.
    * @param \Psr\Log\LoggerInterface $logger
    *   The file logger channel.
    */
-  public function __construct(Settings $settings, LoggerInterface $logger) {
+  public function __construct(StreamWrapperManagerInterface $stream_wrapper_manager, Settings $settings, LoggerInterface $logger) {
+    $this->streamWrapperManager = $stream_wrapper_manager;
     $this->settings = $settings;
     $this->logger = $logger;
   }
 
   /**
-   * Moves an uploaded file to a new location.
-   *
-   * PHP's move_uploaded_file() does not properly support streams if
-   * open_basedir is enabled, so this function fills that gap.
-   *
-   * Compatibility: normal paths and stream wrappers.
-   *
-   * @param string $filename
-   *   The filename of the uploaded file.
-   * @param string $uri
-   *   A string containing the destination URI of the file.
-   *
-   * @return bool
-   *   TRUE on success, or FALSE on failure.
-   *
-   * @see move_uploaded_file()
-   * @see http://drupal.org/node/515192
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function moveUploadedFile($filename, $uri) {
     $result = @move_uploaded_file($filename, $uri);
@@ -90,38 +84,15 @@ public function moveUploadedFile($filename, $uri) {
   }
 
   /**
-   * Sets the permissions on a file or directory.
-   *
-   * This function will use the file_chmod_directory and
-   * file_chmod_file settings for the default modes for directories
-   * and uploaded/generated files. By default these will give everyone read
-   * access so that users accessing the files with a user account without the
-   * webserver group (e.g. via FTP) can read these files, and give group write
-   * permissions so webserver group members (e.g. a vhost account) can alter
-   * files uploaded and owned by the webserver.
-   *
-   * PHP's chmod does not support stream wrappers so we use our wrapper
-   * implementation which interfaces with chmod() by default. Contrib wrappers
-   * may override this behavior in their implementations as needed.
-   *
-   * @param $uri
-   *   A string containing a URI file, or directory path.
-   * @param $mode
-   *   Integer value for the permissions. Consult PHP chmod() documentation for
-   *   more information.
-   *
-   * @return bool
-   *   TRUE for success, FALSE in the event of an error.
-   *
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function chmod($uri, $mode = NULL) {
     if (!isset($mode)) {
       if (is_dir($uri)) {
-        $mode = $this->getSetting('file_chmod_directory', static::CHMOD_DIRECTORY);
+        $mode = $this->settings->get('file_chmod_directory', static::CHMOD_DIRECTORY);
       }
       else {
-        $mode = $this->getSetting('file_chmod_file', static::CHMOD_FILE);
+        $mode = $this->settings->get('file_chmod_file', static::CHMOD_FILE);
       }
     }
 
@@ -134,21 +105,7 @@ public function chmod($uri, $mode = NULL) {
   }
 
   /**
-   * Deletes a file.
-   *
-   * PHP's unlink() is broken on Windows, as it can fail to remove a file when
-   * it has a read-only flag set.
-   *
-   * @param string $uri
-   *   A URI or pathname.
-   * @param $context
-   *   Refer to http://php.net/manual/ref.stream.php
-   *
-   * @return bool
-   *   Boolean TRUE on success, or FALSE on failure.
-   *
-   * @see unlink()
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function unlink($uri, $context = NULL) {
     $scheme = $this->uriScheme($uri);
@@ -164,31 +121,13 @@ public function unlink($uri, $context = NULL) {
   }
 
   /**
-   * Resolves the absolute filepath of a local URI or filepath.
-   *
-   * The use of this method is discouraged, because it does not work for
-   * remote URIs. Except in rare cases, URIs should not be manually resolved.
-   *
-   * Only use this function if you know that the stream wrapper in the URI uses
-   * the local file system, and you need to pass an absolute path to a function
-   * that is incompatible with stream URIs.
-   *
-   * @param string $uri
-   *   A stream wrapper URI or a filepath, possibly including one or more
-   *   symbolic links.
-   *
-   * @return string|false
-   *   The absolute local filepath (with no symbolic links) or FALSE on failure.
-   *
-   * @see \Drupal\Core\StreamWrapper\StreamWrapperInterface::realpath()
-   * @see http://php.net/manual/function.realpath.php
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function realpath($uri) {
     // If this URI is a stream, pass it off to the appropriate stream wrapper.
     // Otherwise, attempt PHP's realpath. This allows use of this method even
     // for unmanaged files outside of the stream wrapper interface.
-    if ($wrapper = $this->getStreamWrapperByUri($uri)) {
+    if ($wrapper = $this->streamWrapperManager->getViaUri($uri)) {
       return $wrapper->realpath();
     }
 
@@ -196,29 +135,13 @@ public function realpath($uri) {
   }
 
   /**
-   * Gets the name of the directory from a given path.
-   *
-   * PHP's dirname() does not properly pass streams, so this function fills that
-   * gap. It is backwards compatible with normal paths and will use PHP's
-   * dirname() as a fallback.
-   *
-   * Compatibility: normal paths and stream wrappers.
-   *
-   * @param string $uri
-   *   A URI or path.
-   *
-   * @return string
-   *   A string containing the directory name.
-   *
-   * @see dirname()
-   * @see http://drupal.org/node/515192
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function dirname($uri) {
     $scheme = $this->uriScheme($uri);
 
     if ($this->validScheme($scheme)) {
-      return $this->getStreamWrapperByScheme($scheme)->dirname($uri);
+      return $this->streamWrapperManager->getViaScheme($scheme)->dirname($uri);
     }
     else {
       return dirname($uri);
@@ -226,15 +149,7 @@ public function dirname($uri) {
   }
 
   /**
-   * Gets the filename from a given path.
-   *
-   * PHP's basename() does not properly support streams or filenames beginning
-   * with a non-US-ASCII character.
-   *
-   * @see http://bugs.php.net/bug.php?id=37738
-   * @see basename()
-   *
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function basename($uri, $suffix = NULL) {
     $separators = '/';
@@ -255,37 +170,11 @@ public function basename($uri, $suffix = NULL) {
   }
 
   /**
-   * Creates a directory, optionally creating missing components in the path to
-   * the directory.
-   *
-   * When PHP's mkdir() creates a directory, the requested mode is affected by
-   * the process's umask. This function overrides the umask and sets the mode
-   * explicitly for all directory components created.
-   *
-   * @param $uri
-   *   A URI or pathname.
-   * @param $mode
-   *   Mode given to created directories. Defaults to the directory mode
-   *   configured in the Drupal installation. It must have a leading zero.
-   * @param $recursive
-   *   Create directories recursively, defaults to FALSE. Cannot work with a
-   *   mode which denies writing or execution to the owner of the process.
-   * @param $context
-   *   Refer to http://php.net/manual/ref.stream.php
-   *
-   * @return bool
-   *   Boolean TRUE on success, or FALSE on failure.
-   *
-   * @see mkdir()
-   * @see http://drupal.org/node/515192
-   * @ingroup php_wrappers
-   *
-   * @todo Update with open_basedir compatible recursion logic from
-   *   \Drupal\Component\PhpStorage\FileStorage::ensureDirectory().
+   * {@inheritdoc}
    */
   public function mkdir($uri, $mode = NULL, $recursive = FALSE, $context = NULL) {
     if (!isset($mode)) {
-      $mode = $this->getSetting('file_chmod_directory', static::CHMOD_DIRECTORY);
+      $mode = $this->settings->get('file_chmod_directory', static::CHMOD_DIRECTORY);
     }
 
     // If the URI has a scheme, don't override the umask - schemes can handle
@@ -356,21 +245,7 @@ protected function mkdirCall($uri, $mode, $recursive, $context) {
   }
 
   /**
-   * Removes a directory.
-   *
-   * PHP's rmdir() is broken on Windows, as it can fail to remove a directory
-   * when it has a read-only flag set.
-   *
-   * @param $uri
-   *   A URI or pathname.
-   * @param $context
-   *   Refer to http://php.net/manual/ref.stream.php
-   *
-   * @return bool
-   *   Boolean TRUE on success, or FALSE on failure.
-   *
-   * @see rmdir()
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function rmdir($uri, $context = NULL) {
     $scheme = $this->uriScheme($uri);
@@ -386,32 +261,13 @@ public function rmdir($uri, $context = NULL) {
   }
 
   /**
-   * Creates a file with a unique filename in the specified directory.
-   *
-   * PHP's tempnam() does not return a URI like we want. This function will
-   * return a URI if given a URI, or it will return a filepath if given a
-   * filepath.
-   *
-   * Compatibility: normal paths and stream wrappers.
-   *
-   * @param $directory
-   *   The directory where the temporary filename will be created.
-   * @param $prefix
-   *   The prefix of the generated temporary filename.
-   *   Note: Windows uses only the first three characters of prefix.
-   *
-   * @return string|bool
-   *   The new temporary filename, or FALSE on failure.
-   *
-   * @see tempnam()
-   * @see http://drupal.org/node/515192
-   * @ingroup php_wrappers
+   * {@inheritdoc}
    */
   public function tempnam($directory, $prefix) {
     $scheme = $this->uriScheme($directory);
 
     if ($this->validScheme($scheme)) {
-      $wrapper = $this->getStreamWrapperByScheme($scheme);
+      $wrapper = $this->streamWrapperManager->getViaScheme($scheme);
 
       if ($filename = tempnam($wrapper->getDirectoryPath(), $prefix)) {
         return $scheme . '://' . static::basename($filename);
@@ -427,16 +283,7 @@ public function tempnam($directory, $prefix) {
   }
 
   /**
-   * Returns the scheme of a URI (e.g. a stream).
-   *
-   * @param string $uri
-   *   A stream, referenced as "scheme://target" or "data:target".
-   *
-   * @return string|bool
-   *   A string containing the name of the scheme, or FALSE if none. For
-   *   example, the URI "public://example.txt" would return "public".
-   *
-   * @see file_uri_target()
+   * {@inheritdoc}
    */
   public function uriScheme($uri) {
     if (preg_match('/^([\w\-]+):\/\/|^(data):/', $uri, $matches)) {
@@ -448,60 +295,13 @@ public function uriScheme($uri) {
   }
 
   /**
-   * Checks that the scheme of a stream URI is valid.
-   *
-   * Confirms that there is a registered stream handler for the provided scheme
-   * and that it is callable. This is useful if you want to confirm a valid
-   * scheme without creating a new instance of the registered handler.
-   *
-   * @param $scheme
-   *   A URI scheme, a stream is referenced as "scheme://target".
-   *
-   * @return bool
-   *   Returns TRUE if the string is the name of a validated stream, or FALSE if
-   *   the scheme does not have a registered handler.
+   * {@inheritdoc}
    */
   public function validScheme($scheme) {
     if (!$scheme) {
       return FALSE;
     }
-    return class_exists($this->getStreamWrapperClass($scheme));
-  }
-
-  /**
-   * Wraps file_stream_wrapper_get_class().
-   *
-   * @codeCoverageIgnore
-   */
-  protected function getStreamWrapperClass($scheme) {
-    return file_stream_wrapper_get_class($scheme);
-  }
-
-  /**
-   * Wraps file_stream_wrapper_get_instance_by_scheme().
-   *
-   * @codeCoverageIgnore
-   */
-  protected function getStreamWrapperByScheme($scheme) {
-    return file_stream_wrapper_get_instance_by_scheme($scheme);
-  }
-
-  /**
-   * Wraps file_stream_wrapper_get_instance_by_uri().
-   *
-   * @codeCoverageIgnore
-   */
-  protected function getStreamWrapperByUri($uri) {
-    return file_stream_wrapper_get_instance_by_uri($uri);
-  }
-
-  /**
-   * Wraps the global Settings singleton.
-   *
-   * @codeCoverageIgnore
-   */
-  protected function getSetting($name, $default = NULL) {
-    return $this->settings->get($name, $default);
+    return class_exists($this->streamWrapperManager->getClass($scheme));
   }
 
 }
diff --git a/core/lib/Drupal/Core/File/FileSystemInterface.php b/core/lib/Drupal/Core/File/FileSystemInterface.php
new file mode 100644
index 0000000..5700903
--- /dev/null
+++ b/core/lib/Drupal/Core/File/FileSystemInterface.php
@@ -0,0 +1,245 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\File\FileSystemInterface.
+ */
+
+namespace Drupal\Core\File;
+
+/**
+ * Provides an interface for helpers that operate on files and stream wrappers.
+ */
+interface FileSystemInterface {
+
+  /**
+   * Moves an uploaded file to a new location.
+   *
+   * PHP's move_uploaded_file() does not properly support streams if
+   * open_basedir is enabled, so this function fills that gap.
+   *
+   * Compatibility: normal paths and stream wrappers.
+   *
+   * @param string $filename
+   *   The filename of the uploaded file.
+   * @param string $uri
+   *   A string containing the destination URI of the file.
+   *
+   * @return bool
+   *   TRUE on success, or FALSE on failure.
+   *
+   * @see move_uploaded_file()
+   * @see http://drupal.org/node/515192
+   * @ingroup php_wrappers
+   */
+  public function moveUploadedFile($filename, $uri);
+
+  /**
+   * Sets the permissions on a file or directory.
+   *
+   * This function will use the file_chmod_directory and
+   * file_chmod_file settings for the default modes for directories
+   * and uploaded/generated files. By default these will give everyone read
+   * access so that users accessing the files with a user account without the
+   * webserver group (e.g. via FTP) can read these files, and give group write
+   * permissions so webserver group members (e.g. a vhost account) can alter
+   * files uploaded and owned by the webserver.
+   *
+   * PHP's chmod does not support stream wrappers so we use our wrapper
+   * implementation which interfaces with chmod() by default. Contrib wrappers
+   * may override this behavior in their implementations as needed.
+   *
+   * @param string $uri
+   *   A string containing a URI file, or directory path.
+   * @param int $mode
+   *   Integer value for the permissions. Consult PHP chmod() documentation for
+   *   more information.
+   *
+   * @return bool
+   *   TRUE for success, FALSE in the event of an error.
+   *
+   * @ingroup php_wrappers
+   */
+  public function chmod($uri, $mode = NULL);
+
+  /**
+   * Deletes a file.
+   *
+   * PHP's unlink() is broken on Windows, as it can fail to remove a file when
+   * it has a read-only flag set.
+   *
+   * @param string $uri
+   *   A URI or pathname.
+   * @param resource $context
+   *   Refer to http://php.net/manual/ref.stream.php
+   *
+   * @return bool
+   *   Boolean TRUE on success, or FALSE on failure.
+   *
+   * @see unlink()
+   * @ingroup php_wrappers
+   */
+  public function unlink($uri, $context = NULL);
+
+  /**
+   * Resolves the absolute filepath of a local URI or filepath.
+   *
+   * The use of this method is discouraged, because it does not work for
+   * remote URIs. Except in rare cases, URIs should not be manually resolved.
+   *
+   * Only use this function if you know that the stream wrapper in the URI uses
+   * the local file system, and you need to pass an absolute path to a function
+   * that is incompatible with stream URIs.
+   *
+   * @param string $uri
+   *   A stream wrapper URI or a filepath, possibly including one or more
+   *   symbolic links.
+   *
+   * @return string|false
+   *   The absolute local filepath (with no symbolic links) or FALSE on failure.
+   *
+   * @see \Drupal\Core\StreamWrapper\StreamWrapperInterface::realpath()
+   * @see http://php.net/manual/function.realpath.php
+   * @ingroup php_wrappers
+   */
+  public function realpath($uri);
+
+  /**
+   * Gets the name of the directory from a given path.
+   *
+   * PHP's dirname() does not properly pass streams, so this function fills that
+   * gap. It is backwards compatible with normal paths and will use PHP's
+   * dirname() as a fallback.
+   *
+   * Compatibility: normal paths and stream wrappers.
+   *
+   * @param string $uri
+   *   A URI or path.
+   *
+   * @return string
+   *   A string containing the directory name.
+   *
+   * @see dirname()
+   * @see http://drupal.org/node/515192
+   * @ingroup php_wrappers
+   */
+  public function dirname($uri);
+
+  /**
+   * Gets the filename from a given path.
+   *
+   * PHP's basename() does not properly support streams or filenames beginning
+   * with a non-US-ASCII character.
+   *
+   * @see http://bugs.php.net/bug.php?id=37738
+   * @see basename()
+   *
+   * @ingroup php_wrappers
+   */
+  public function basename($uri, $suffix = NULL);
+
+  /**
+   * Creates a directory, optionally creating missing components in the path to
+   * the directory.
+   *
+   * When PHP's mkdir() creates a directory, the requested mode is affected by
+   * the process's umask. This function overrides the umask and sets the mode
+   * explicitly for all directory components created.
+   *
+   * @param string $uri
+   *   A URI or pathname.
+   * @param int $mode
+   *   Mode given to created directories. Defaults to the directory mode
+   *   configured in the Drupal installation. It must have a leading zero.
+   * @param bool $recursive
+   *   Create directories recursively, defaults to FALSE. Cannot work with a
+   *   mode which denies writing or execution to the owner of the process.
+   * @param resource $context
+   *   Refer to http://php.net/manual/ref.stream.php
+   *
+   * @return bool
+   *   Boolean TRUE on success, or FALSE on failure.
+   *
+   * @see mkdir()
+   * @see http://drupal.org/node/515192
+   * @ingroup php_wrappers
+   *
+   * @todo Update with open_basedir compatible recursion logic from
+   *   \Drupal\Component\PhpStorage\FileStorage::ensureDirectory().
+   */
+  public function mkdir($uri, $mode = NULL, $recursive = FALSE, $context = NULL);
+
+  /**
+   * Removes a directory.
+   *
+   * PHP's rmdir() is broken on Windows, as it can fail to remove a directory
+   * when it has a read-only flag set.
+   *
+   * @param string $uri
+   *   A URI or pathname.
+   * @param resource $context
+   *   Refer to http://php.net/manual/ref.stream.php
+   *
+   * @return bool
+   *   Boolean TRUE on success, or FALSE on failure.
+   *
+   * @see rmdir()
+   * @ingroup php_wrappers
+   */
+  public function rmdir($uri, $context = NULL);
+
+  /**
+   * Creates a file with a unique filename in the specified directory.
+   *
+   * PHP's tempnam() does not return a URI like we want. This function will
+   * return a URI if given a URI, or it will return a filepath if given a
+   * filepath.
+   *
+   * Compatibility: normal paths and stream wrappers.
+   *
+   * @param string $directory
+   *   The directory where the temporary filename will be created.
+   * @param string $prefix
+   *   The prefix of the generated temporary filename.
+   *   Note: Windows uses only the first three characters of prefix.
+   *
+   * @return string|bool
+   *   The new temporary filename, or FALSE on failure.
+   *
+   * @see tempnam()
+   * @see http://drupal.org/node/515192
+   * @ingroup php_wrappers
+   */
+  public function tempnam($directory, $prefix);
+
+  /**
+   * Returns the scheme of a URI (e.g. a stream).
+   *
+   * @param string $uri
+   *   A stream, referenced as "scheme://target" or "data:target".
+   *
+   * @return string|bool
+   *   A string containing the name of the scheme, or FALSE if none. For
+   *   example, the URI "public://example.txt" would return "public".
+   *
+   * @see file_uri_target()
+   */
+  public function uriScheme($uri);
+
+  /**
+   * Checks that the scheme of a stream URI is valid.
+   *
+   * Confirms that there is a registered stream handler for the provided scheme
+   * and that it is callable. This is useful if you want to confirm a valid
+   * scheme without creating a new instance of the registered handler.
+   *
+   * @param string $scheme
+   *   A URI scheme, a stream is referenced as "scheme://target".
+   *
+   * @return bool
+   *   Returns TRUE if the string is the name of a validated stream, or FALSE if
+   *   the scheme does not have a registered handler.
+   */
+  public function validScheme($scheme);
+
+}
diff --git a/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php b/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php
index 101980d..dc9f4b6 100644
--- a/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php
+++ b/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\StreamWrapper;
 
-use Drupal\Core\Extension\ModuleHandlerInterface;
 use Symfony\Component\DependencyInjection\ContainerAware;
 
 /**
@@ -16,7 +15,7 @@
  * @see file_get_stream_wrappers()
  * @see \Drupal\Core\StreamWrapper\StreamWrapperInterface
  */
-class StreamWrapperManager extends ContainerAware {
+class StreamWrapperManager extends ContainerAware implements StreamWrapperManagerInterface {
 
   /**
    * Contains stream wrapper info.
@@ -48,57 +47,7 @@ class StreamWrapperManager extends ContainerAware {
   protected $wrappers = array();
 
   /**
-   * Provides Drupal stream wrapper registry.
-   *
-   * A stream wrapper is an abstraction of a file system that allows Drupal to
-   * use the same set of methods to access both local files and remote
-   * resources.
-   *
-   * Provide a facility for managing and querying user-defined stream wrappers
-   * in PHP. PHP's internal stream_get_wrappers() doesn't return the class
-   * registered to handle a stream, which we need to be able to find the handler
-   * for class instantiation.
-   *
-   * If a module registers a scheme that is already registered with PHP, the
-   * existing scheme will be unregistered and replaced with the specified class.
-   *
-   * A stream is referenced as "scheme://target".
-   *
-   * The optional $filter parameter can be used to retrieve only the stream
-   * wrappers that are appropriate for particular usage. For example, this
-   * returns only stream wrappers that use local file storage:
-   *
-   * @code
-   *   $local_stream_wrappers = file_get_stream_wrappers(StreamWrapperInterface::LOCAL);
-   * @endcode
-   *
-   * The $filter parameter can only filter to types containing a particular
-   * flag. In some cases, you may want to filter to types that do not contain a
-   * particular flag. For example, you may want to retrieve all stream wrappers
-   * that are not writable, or all stream wrappers that are not local. PHP's
-   * array_diff_key() function can be used to help with this. For example, this
-   * returns only stream wrappers that do not use local file storage:
-   * @code
-   *   $remote_stream_wrappers = array_diff_key(file_get_stream_wrappers(StreamWrapperInterface::ALL), file_get_stream_wrappers(StreamWrapperInterface::LOCAL));
-   * @endcode
-   *
-   * @param int $filter
-   *   (Optional) Filters out all types except those with an on bit for each on
-   *   bit in $filter. For example, if $filter is
-   *   StreamWrapperInterface::WRITE_VISIBLE, which is equal to
-   *   (StreamWrapperInterface::READ | StreamWrapperInterface::WRITE |
-   *   StreamWrapperInterface::VISIBLE), then only stream wrappers with all
-   *   three of these bits set are returned. Defaults to
-   *   StreamWrapperInterface::ALL, which returns all registered stream
-   *   wrappers.
-   *
-   * @return array
-   *   An array keyed by scheme, with values containing an array of information
-   *   about the stream wrapper, as returned by hook_stream_wrappers(). If
-   *   $filter is omitted or set to StreamWrapperInterface::ALL, the entire
-   *   Drupal stream wrapper registry is returned. Otherwise only the stream
-   *   wrappers whose 'type' bitmask has an on bit for each bit specified in
-   *   $filter are returned.
+   * {@inheritdoc}
    */
   public function getWrappers($filter = StreamWrapperInterface::ALL) {
     if (isset($this->wrappers[$filter])) {
@@ -120,20 +69,7 @@ public function getWrappers($filter = StreamWrapperInterface::ALL) {
   }
 
   /**
-   * Returns registered stream wrapper names.
-   *
-   * @param int $filter
-   *   (Optional) Filters out all types except those with an on bit for each on
-   *   bit in $filter. For example, if $filter is
-   *   StreamWrapperInterface::WRITE_VISIBLE, which is equal to
-   *   (StreamWrapperInterface::READ | StreamWrapperInterface::WRITE |
-   *   StreamWrapperInterface::VISIBLE), then only stream wrappers with all
-   *   three of these bits set are returned. Defaults to
-   *   StreamWrapperInterface::ALL, which returns all registered stream
-   *   wrappers.
-   *
-   * @return array
-   *   Stream wrapper names, keyed by scheme.
+   * {@inheritdoc}
    */
   public function getNames($filter = StreamWrapperInterface::ALL) {
     $names = array();
@@ -145,20 +81,7 @@ public function getNames($filter = StreamWrapperInterface::ALL) {
   }
 
   /**
-   * Returns registered stream wrapper descriptions.
-   *
-   * @param int $filter
-   *   (Optional) Filters out all types except those with an on bit for each on
-   *   bit in $filter. For example, if $filter is
-   *   StreamWrapperInterface::WRITE_VISIBLE, which is equal to
-   *   (StreamWrapperInterface::READ | StreamWrapperInterface::WRITE |
-   *   StreamWrapperInterface::VISIBLE), then only stream wrappers with all
-   *   three of these bits set are returned. Defaults to
-   *   StreamWrapperInterface::ALL, which returns all registered stream
-   *   wrappers.
-   *
-   * @return array
-   *   Stream wrapper descriptions, keyed by scheme.
+   * {@inheritdoc}
    */
   public function getDescriptions($filter = StreamWrapperInterface::ALL) {
     $descriptions = array();
@@ -170,26 +93,14 @@ public function getDescriptions($filter = StreamWrapperInterface::ALL) {
   }
 
   /**
-   * Returns a stream wrapper via scheme.
-   *
-   * @param string $scheme
-   *   The scheme of the stream wrapper.
-   *
-   * @return \Drupal\Core\StreamWrapper\StreamWrapperInterface|bool
-   *   A stream wrapper object, or false if the scheme is not available.
+   * {@inheritdoc}
    */
   public function getViaScheme($scheme) {
     return $this->getWrapper($scheme, $scheme . '://');
   }
 
   /**
-   * Returns a stream wrapper via URI.
-   *
-   * @param string $uri
-   *   The URI of the stream wrapper.
-   *
-   * @return \Drupal\Core\StreamWrapper\StreamWrapperInterface|bool
-   *   A stream wrapper object, or false if the scheme is not available.
+   * {@inheritdoc}
    */
   public function getViaUri($uri) {
     $scheme = file_uri_scheme($uri);
@@ -197,13 +108,7 @@ public function getViaUri($uri) {
   }
 
   /**
-   * Returns the stream wrapper class.
-   *
-   * @param string $scheme
-   *   The stream wrapper scheme.
-   *
-   * @return string|bool
-   *   The stream wrapper class, or false if the scheme does not exist.
+   * {@inheritdoc}
    */
   public function getClass($scheme) {
     if (isset($this->info[$scheme])) {
@@ -283,14 +188,7 @@ public function unregister() {
   }
 
   /**
-   * Registers stream wrapper with PHP.
-   *
-   * @param string $scheme
-   *   The scheme of the stream wrapper.
-   * @param string $class
-   *   The class of the stream wrapper.
-   * @param int $type
-   *   The type of the stream wrapper.
+   * {@inheritdoc}
    */
   public function registerWrapper($scheme, $class, $type) {
     if (in_array($scheme, stream_get_wrappers(), TRUE)) {
diff --git a/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManagerInterface.php b/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManagerInterface.php
new file mode 100644
index 0000000..34e5f81
--- /dev/null
+++ b/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManagerInterface.php
@@ -0,0 +1,159 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface.
+ */
+
+namespace Drupal\Core\StreamWrapper;
+
+/**
+ * Provides a StreamWrapper manager.
+ *
+ * @see file_get_stream_wrappers()
+ * @see \Drupal\Core\StreamWrapper\StreamWrapperInterface
+ */
+interface StreamWrapperManagerInterface {
+
+  /**
+   * Provides Drupal stream wrapper registry.
+   *
+   * A stream wrapper is an abstraction of a file system that allows Drupal to
+   * use the same set of methods to access both local files and remote
+   * resources.
+   *
+   * Provide a facility for managing and querying user-defined stream wrappers
+   * in PHP. PHP's internal stream_get_wrappers() doesn't return the class
+   * registered to handle a stream, which we need to be able to find the
+   * handler
+   * for class instantiation.
+   *
+   * If a module registers a scheme that is already registered with PHP, the
+   * existing scheme will be unregistered and replaced with the specified
+   * class.
+   *
+   * A stream is referenced as "scheme://target".
+   *
+   * The optional $filter parameter can be used to retrieve only the stream
+   * wrappers that are appropriate for particular usage. For example, this
+   * returns only stream wrappers that use local file storage:
+   *
+   * @code
+   *   $local_stream_wrappers =
+   *   file_get_stream_wrappers(StreamWrapperInterface::LOCAL);
+   * @endcode
+   *
+   * The $filter parameter can only filter to types containing a particular
+   * flag. In some cases, you may want to filter to types that do not contain a
+   * particular flag. For example, you may want to retrieve all stream wrappers
+   * that are not writable, or all stream wrappers that are not local. PHP's
+   * array_diff_key() function can be used to help with this. For example, this
+   * returns only stream wrappers that do not use local file storage:
+   * @code
+   *   $remote_stream_wrappers =
+   *   array_diff_key(file_get_stream_wrappers(StreamWrapperInterface::ALL),
+   *   file_get_stream_wrappers(StreamWrapperInterface::LOCAL));
+   * @endcode
+   *
+   * @param int $filter
+   *   (Optional) Filters out all types except those with an on bit for each on
+   *   bit in $filter. For example, if $filter is
+   *   StreamWrapperInterface::WRITE_VISIBLE, which is equal to
+   *   (StreamWrapperInterface::READ | StreamWrapperInterface::WRITE |
+   *   StreamWrapperInterface::VISIBLE), then only stream wrappers with all
+   *   three of these bits set are returned. Defaults to
+   *   StreamWrapperInterface::ALL, which returns all registered stream
+   *   wrappers.
+   *
+   * @return array
+   *   An array keyed by scheme, with values containing an array of information
+   *   about the stream wrapper, as returned by hook_stream_wrappers(). If
+   *   $filter is omitted or set to StreamWrapperInterface::ALL, the entire
+   *   Drupal stream wrapper registry is returned. Otherwise only the stream
+   *   wrappers whose 'type' bitmask has an on bit for each bit specified in
+   *   $filter are returned.
+   */
+  public function getWrappers($filter = StreamWrapperInterface::ALL);
+
+  /**
+   * Returns registered stream wrapper names.
+   *
+   * @param int $filter
+   *   (Optional) Filters out all types except those with an on bit for each on
+   *   bit in $filter. For example, if $filter is
+   *   StreamWrapperInterface::WRITE_VISIBLE, which is equal to
+   *   (StreamWrapperInterface::READ | StreamWrapperInterface::WRITE |
+   *   StreamWrapperInterface::VISIBLE), then only stream wrappers with all
+   *   three of these bits set are returned. Defaults to
+   *   StreamWrapperInterface::ALL, which returns all registered stream
+   *   wrappers.
+   *
+   * @return array
+   *   Stream wrapper names, keyed by scheme.
+   */
+  public function getNames($filter = StreamWrapperInterface::ALL);
+
+  /**
+   * Returns registered stream wrapper descriptions.
+   *
+   * @param int $filter
+   *   (Optional) Filters out all types except those with an on bit for each on
+   *   bit in $filter. For example, if $filter is
+   *   StreamWrapperInterface::WRITE_VISIBLE, which is equal to
+   *   (StreamWrapperInterface::READ | StreamWrapperInterface::WRITE |
+   *   StreamWrapperInterface::VISIBLE), then only stream wrappers with all
+   *   three of these bits set are returned. Defaults to
+   *   StreamWrapperInterface::ALL, which returns all registered stream
+   *   wrappers.
+   *
+   * @return array
+   *   Stream wrapper descriptions, keyed by scheme.
+   */
+  public function getDescriptions($filter = StreamWrapperInterface::ALL);
+
+  /**
+   * Returns a stream wrapper via scheme.
+   *
+   * @param string $scheme
+   *   The scheme of the stream wrapper.
+   *
+   * @return \Drupal\Core\StreamWrapper\StreamWrapperInterface|bool
+   *   A stream wrapper object, or false if the scheme is not available.
+   */
+  public function getViaScheme($scheme);
+
+  /**
+   * Returns a stream wrapper via URI.
+   *
+   * @param string $uri
+   *   The URI of the stream wrapper.
+   *
+   * @return \Drupal\Core\StreamWrapper\StreamWrapperInterface|bool
+   *   A stream wrapper object, or false if the scheme is not available.
+   */
+  public function getViaUri($uri);
+
+  /**
+   * Returns the stream wrapper class.
+   *
+   * @param string $scheme
+   *   The stream wrapper scheme.
+   *
+   * @return string|bool
+   *   The stream wrapper class, or false if the scheme does not exist.
+   */
+  public function getClass($scheme);
+
+  /**
+   * Registers stream wrapper with PHP.
+   *
+   * @param string $scheme
+   *   The scheme of the stream wrapper.
+   * @param string $class
+   *   The class of the stream wrapper.
+   * @param int $type
+   *   The type of the stream wrapper.
+   */
+  public function registerWrapper($scheme, $class, $type);
+
+}
diff --git a/core/modules/system/src/Form/FileSystemForm.php b/core/modules/system/src/Form/FileSystemForm.php
index 368f318..40bd937 100644
--- a/core/modules/system/src/Form/FileSystemForm.php
+++ b/core/modules/system/src/Form/FileSystemForm.php
@@ -15,7 +15,7 @@
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\StreamWrapper\StreamWrapperInterface;
-use Drupal\Core\StreamWrapper\StreamWrapperManager;
+use Drupal\Core\StreamWrapper\StreamWrapperManagerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -33,7 +33,7 @@ class FileSystemForm extends ConfigFormBase {
   /**
    * The stream wrapper manager.
    *
-   * @var \Drupal\Core\StreamWrapper\StreamWrapperManager
+   * @var \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface
    */
   protected $streamWrapperManager;
 
@@ -44,10 +44,10 @@ class FileSystemForm extends ConfigFormBase {
    *   The factory for configuration objects.
    * @param \Drupal\Core\Datetime\DateFormatter $date_formatter
    *   The date formatter service.
-   * @param \Drupal\Core\StreamWrapper\StreamWrapperManager $stream_wrapper_manager
+   * @param \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface $stream_wrapper_manager
    *   The stream wrapper manager.
    */
-  public function __construct(ConfigFactoryInterface $config_factory, DateFormatter $date_formatter, StreamWrapperManager $stream_wrapper_manager) {
+  public function __construct(ConfigFactoryInterface $config_factory, DateFormatter $date_formatter, StreamWrapperManagerInterface $stream_wrapper_manager) {
     parent::__construct($config_factory);
     $this->dateFormatter = $date_formatter;
     $this->streamWrapperManager = $stream_wrapper_manager;
diff --git a/core/tests/Drupal/Tests/Core/File/FileSystemTest.php b/core/tests/Drupal/Tests/Core/File/FileSystemTest.php
index da854f7..ba39d17 100644
--- a/core/tests/Drupal/Tests/Core/File/FileSystemTest.php
+++ b/core/tests/Drupal/Tests/Core/File/FileSystemTest.php
@@ -38,8 +38,9 @@ protected function setUp() {
     parent::setUp();
 
     $settings = new Settings([]);
+    $stream_wrapper_manager = $this->getMock('Drupal\Core\StreamWrapper\StreamWrapperManagerInterface');
     $this->logger = $this->getMock('Psr\Log\LoggerInterface');
-    $this->fileSystem = new FileSystem($settings, $this->logger);
+    $this->fileSystem = new FileSystem($stream_wrapper_manager, $settings, $this->logger);
   }
 
   /**