diff -u b/core/modules/node/src/Tests/PagePreviewTest.php b/core/modules/node/src/Tests/PagePreviewTest.php
--- b/core/modules/node/src/Tests/PagePreviewTest.php
+++ b/core/modules/node/src/Tests/PagePreviewTest.php
@@ -316,11 +316,13 @@
     $this->assertUrl($node->toUrl());
     $this->assertResponse(200);
 
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     // Assert multiple items can be added and are not lost when previewing.
     $test_image_1 = current($this->drupalGetTestFiles('image', 39325));
-    $edit_image_1['files[field_image_0][]'] = drupal_realpath($test_image_1->uri);
+    $edit_image_1['files[field_image_0][]'] = $file_system->realpath($test_image_1->uri);
     $test_image_2 = current($this->drupalGetTestFiles('image', 39325));
-    $edit_image_2['files[field_image_1][]'] = drupal_realpath($test_image_2->uri);
+    $edit_image_2['files[field_image_1][]'] = $file_system->realpath($test_image_2->uri);
     $edit['field_image[0][alt]'] = 'Alt 1';
 
     $this->drupalPostForm('node/add/page', $edit_image_1, t('Upload'));
only in patch2:
unchanged:
--- a/core/modules/file/src/Tests/FilePrivateTest.php
+++ b/core/modules/file/src/Tests/FilePrivateTest.php
@@ -38,6 +38,8 @@ protected function setUp() {
    */
   public function testPrivateFile() {
     $node_storage = $this->container->get('entity.manager')->getStorage('node');
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $type_name = 'article';
     $field_name = strtolower($this->randomMachineName());
     $this->createFileField($field_name, 'node', $type_name, ['uri_scheme' => 'private']);
@@ -128,7 +130,7 @@ public function testPrivateFile() {
     );
     $test_file = $this->getTestFile('text');
     $this->drupalGet('node/add/' . $type_name);
-    $edit = ['files[' . $field_name . '_0]' => drupal_realpath($test_file->getFileUri())];
+    $edit = ['files[' . $field_name . '_0]' => $file_system->realpath($test_file->getFileUri())];
     $this->drupalPostForm(NULL, $edit, t('Upload'));
     /** @var \Drupal\file\FileStorageInterface $file_storage */
     $file_storage = $this->container->get('entity.manager')->getStorage('file');
@@ -155,7 +157,7 @@ public function testPrivateFile() {
     $this->drupalGet('node/add/' . $type_name);
     $edit = [];
     $edit['title[0][value]'] = $this->randomMachineName();
-    $edit['files[' . $field_name . '_0]'] = drupal_realpath($test_file->getFileUri());
+    $edit['files[' . $field_name . '_0]'] = $file_system->realpath($test_file->getFileUri());
     $this->drupalPostForm(NULL, $edit, t('Save'));
     $new_node = $this->drupalGetNodeByTitle($edit['title[0][value]']);
     $file_id = $new_node->{$field_name}->target_id;
@@ -184,7 +186,7 @@ public function testPrivateFile() {
     $this->drupalGet('node/add/' . $type_name);
     $edit = [];
     $edit['title[0][value]'] = $this->randomMachineName();
-    $edit['files[' . $field_name . '_0]'] = drupal_realpath($test_file->getFileUri());
+    $edit['files[' . $field_name . '_0]'] = $file_system->realpath($test_file->getFileUri());
     $this->drupalPostForm(NULL, $edit, t('Save'));
     $new_node = $this->drupalGetNodeByTitle($edit['title[0][value]']);
     $file = File::load($new_node->{$field_name}->target_id);
@@ -209,7 +211,7 @@ public function testPrivateFile() {
     $this->drupalGet('node/add/' . $type_name);
     $edit = [];
     $edit['title[0][value]'] = $this->randomMachineName();
-    $edit['files[' . $field_name . '_0]'] = drupal_realpath($test_file->getFileUri());
+    $edit['files[' . $field_name . '_0]'] = $file_system->realpath($test_file->getFileUri());
     $this->drupalPostForm(NULL, $edit, t('Save'));
     $new_node = $this->drupalGetNodeByTitle($edit['title[0][value]']);
     $new_node->setPublished(FALSE);
only in patch2:
unchanged:
--- a/core/modules/file/src/Tests/SaveUploadFormTest.php
+++ b/core/modules/file/src/Tests/SaveUploadFormTest.php
@@ -63,10 +63,12 @@ protected function setUp() {
 
     $this->maxFidBefore = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
 
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     // Upload with replace to guarantee there's something there.
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertResponse(200, 'Received a 200 response for posted test file.');
@@ -94,7 +96,9 @@ public function testNormal() {
 
     // Upload a second file.
     $image2 = current($this->drupalGetTestFiles('image'));
-    $edit = ['files[file_test_upload][]' => drupal_realpath($image2->uri)];
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
+    $edit = ['files[file_test_upload][]' => $file_system->realpath($image2->uri)];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertResponse(200, 'Received a 200 response for posted test file.');
     $this->assertRaw(t('You WIN!'));
@@ -115,7 +119,7 @@ public function testNormal() {
 
     // Upload a third file to a subdirectory.
     $image3 = current($this->drupalGetTestFiles('image'));
-    $image3_realpath = drupal_realpath($image3->uri);
+    $image3_realpath = $file_system->realpath($image3->uri);
     $dir = $this->randomMachineName();
     $edit = [
       'files[file_test_upload][]' => $image3_realpath,
@@ -131,6 +135,8 @@ public function testNormal() {
    * Tests extension handling.
    */
   public function testHandleExtension() {
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     // The file being tested is a .gif which is in the default safe list
     // of extensions to allow when the extension validator isn't used. This is
     // implicitly tested at the testNormal() test. Here we tell
@@ -138,7 +144,7 @@ public function testHandleExtension() {
     $extensions = 'foo';
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'extensions' => $extensions,
     ];
 
@@ -158,7 +164,7 @@ public function testHandleExtension() {
     // Now tell _file_save_upload_from_form() to allow the extension of our test image.
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'extensions' => $extensions,
     ];
 
@@ -176,7 +182,7 @@ public function testHandleExtension() {
     // Now tell _file_save_upload_from_form() to allow any extension.
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'allow_all_extensions' => TRUE,
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
@@ -193,11 +199,13 @@ public function testHandleExtension() {
    */
   public function testHandleDangerousFile() {
     $config = $this->config('system.file');
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     // Allow the .php extension and make sure it gets renamed to .txt for
     // safety. Also check to make sure its MIME type was changed.
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload][]' => drupal_realpath($this->phpfile->uri),
+      'files[file_test_upload][]' => $file_system->realpath($this->phpfile->uri),
       'is_image_file' => FALSE,
       'extensions' => 'php',
     ];
@@ -235,6 +243,8 @@ public function testHandleDangerousFile() {
    * Tests file munge handling.
    */
   public function testHandleFileMunge() {
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     // Ensure insecure uploads are disabled for this test.
     $this->config('system.file')->set('allow_insecure_uploads', 0)->save();
     $this->image = file_move($this->image, $this->image->getFileUri() . '.foo.' . $this->imageExtension);
@@ -244,7 +254,7 @@ public function testHandleFileMunge() {
 
     $extensions = $this->imageExtension;
     $edit = [
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'extensions' => $extensions,
     ];
 
@@ -266,7 +276,7 @@ public function testHandleFileMunge() {
     file_test_reset();
 
     $edit = [
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'allow_all_extensions' => TRUE,
     ];
 
@@ -284,9 +294,11 @@ public function testHandleFileMunge() {
    * Tests renaming when uploading over a file that already exists.
    */
   public function testExistingRename() {
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
       'file_test_replace' => FILE_EXISTS_RENAME,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri())
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertResponse(200, 'Received a 200 response for posted test file.');
@@ -302,7 +314,7 @@ public function testExistingRename() {
   public function testExistingReplace() {
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri())
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertResponse(200, 'Received a 200 response for posted test file.');
@@ -316,9 +328,11 @@ public function testExistingReplace() {
    * Tests for failure when uploading over a file that already exists.
    */
   public function testExistingError() {
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
       'file_test_replace' => FILE_EXISTS_ERROR,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri())
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertResponse(200, 'Received a 200 response for posted test file.');
@@ -345,9 +359,11 @@ public function testDrupalMovingUploadedFileError() {
     drupal_mkdir('temporary://' . $test_directory, 0000);
     $this->assertTrue(is_dir('temporary://' . $test_directory));
 
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
       'file_subdir' => $test_directory,
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri())
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri())
     ];
 
     \Drupal::state()->set('file_test.disable_error_collection', TRUE);
@@ -371,8 +387,10 @@ public function testDrupalMovingUploadedFileError() {
   public function testErrorMessagesAreNotChanged() {
     $error = 'An error message set before _file_save_upload_from_form()';
 
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'error_message' => $error,
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
@@ -387,7 +405,7 @@ public function testErrorMessagesAreNotChanged() {
 
     // Test that error messages are preserved when an error occurs.
     $edit = [
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'error_message' => $error,
       'extensions' => 'foo'
     ];
@@ -403,7 +421,7 @@ public function testErrorMessagesAreNotChanged() {
 
     // Test a successful upload with no messages.
     $edit = [
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertResponse(200, 'Received a 200 response for posted test file.');
@@ -423,10 +441,12 @@ public function testCombinedErrorMessages() {
     $textfile = current($this->drupalGetTestFiles('text'));
     $this->assertTrue(is_file($textfile->uri), 'The text file we are going to upload exists.');
 
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
       'files[file_test_upload][]' => [
-        drupal_realpath($this->phpfile->uri),
-        drupal_realpath($textfile->uri),
+        $file_system->realpath($this->phpfile->uri),
+        $file_system->realpath($textfile->uri),
       ],
       'allow_all_extensions' => FALSE,
       'is_image_file' => TRUE,
@@ -447,8 +467,10 @@ public function testCombinedErrorMessages() {
   public function testUploadFieldIsHighlighted() {
     $this->assertEqual(0, count($this->cssSelect('input[name="files[file_test_upload][]"].error')), 'Successful file upload has no error.');
 
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
-      'files[file_test_upload][]' => drupal_realpath($this->image->getFileUri()),
+      'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri()),
       'extensions' => 'foo'
     ];
     $this->drupalPostForm('file-test/save_upload_from_form_test', $edit, t('Submit'));
only in patch2:
unchanged:
--- a/core/modules/file/tests/src/Functional/FileFieldTestBase.php
+++ b/core/modules/file/tests/src/Functional/FileFieldTestBase.php
@@ -248,7 +248,7 @@ public function removeNodeFile($nid, $new_revision = TRUE) {
    */
   public function replaceNodeFile($file, $field_name, $nid, $new_revision = TRUE) {
     $edit = [
-      'files[' . $field_name . '_0]' => drupal_realpath($file->getFileUri()),
+      'files[' . $field_name . '_0]' => \Drupal::service('file_system')->realpath($file->getFileUri()),
       'revision' => (string) (int) $new_revision,
     ];
 
only in patch2:
unchanged:
--- a/core/modules/image/tests/src/Functional/ImageFieldTestBase.php
+++ b/core/modules/image/tests/src/Functional/ImageFieldTestBase.php
@@ -66,7 +66,7 @@ public function previewNodeImage($image, $field_name, $type) {
     $edit = [
       'title[0][value]' => $this->randomMachineName(),
     ];
-    $edit['files[' . $field_name . '_0]'] = drupal_realpath($image->uri);
+    $edit['files[' . $field_name . '_0]'] = \Drupal::service('file_system')->realpath($image->uri);
     $this->drupalPostForm('node/add/' . $type, $edit, t('Preview'));
   }
 
@@ -86,7 +86,7 @@ public function uploadNodeImage($image, $field_name, $type, $alt = '') {
     $edit = [
       'title[0][value]' => $this->randomMachineName(),
     ];
-    $edit['files[' . $field_name . '_0]'] = drupal_realpath($image->uri);
+    $edit['files[' . $field_name . '_0]'] = \Drupal::service('file_system')->realpath($image->uri);
     $this->drupalPostForm('node/add/' . $type, $edit, t('Save and publish'));
     if ($alt) {
       // Add alt text.
only in patch2:
unchanged:
--- a/core/tests/Drupal/FunctionalTests/Entity/ContentEntityFormFieldValidationFilteringTest.php
+++ b/core/tests/Drupal/FunctionalTests/Entity/ContentEntityFormFieldValidationFilteringTest.php
@@ -139,7 +139,7 @@ public function testFieldWidgetsWithLimitedValidationErrors() {
     // \Drupal\file\Plugin\Field\FieldWidget\FileWidget::process().
     $text_file = current($this->getTestFiles('text'));
     $edit = [
-      'files[test_file_0]' => drupal_realpath($text_file->uri)
+      'files[test_file_0]' => \Drupal::service('file_system')->realpath($text_file->uri)
     ];
     $assert_session->elementNotExists('css', 'input#edit-test-file-0-remove-button');
     $this->drupalPostForm(NULL, $edit, 'Upload');