diff -u b/core/lib/Drupal/Core/File/Exception/DirectoryNotReadyException.php b/core/lib/Drupal/Core/File/Exception/DirectoryNotReadyException.php
--- b/core/lib/Drupal/Core/File/Exception/DirectoryNotReadyException.php
+++ b/core/lib/Drupal/Core/File/Exception/DirectoryNotReadyException.php
@@ -1,11 +1,12 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\File\Exception\DirectoryNotReadyException.
- */
-
 namespace Drupal\Core\File\Exception;
 
+/**
+ * Exception thrown when a file's destination directory is not ready.
+ *
+ * A directory can be considered not ready when it either does not exist, or
+ * is not writable.
+ */
 class DirectoryNotReadyException extends FileException {
 }
diff -u b/core/lib/Drupal/Core/File/Exception/FileException.php b/core/lib/Drupal/Core/File/Exception/FileException.php
--- b/core/lib/Drupal/Core/File/Exception/FileException.php
+++ b/core/lib/Drupal/Core/File/Exception/FileException.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\File\Exception\FileException.
- */
-
 namespace Drupal\Core\File\Exception;
 
 /**
diff -u b/core/lib/Drupal/Core/File/Exception/FileExistsException.php b/core/lib/Drupal/Core/File/Exception/FileExistsException.php
--- b/core/lib/Drupal/Core/File/Exception/FileExistsException.php
+++ b/core/lib/Drupal/Core/File/Exception/FileExistsException.php
@@ -1,11 +1,9 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\File\Exception\FileExistsException.
- */
-
 namespace Drupal\Core\File\Exception;
 
+/**
+ * Exception thrown when a file unexpectedly exists.
+ */
 class FileExistsException extends FileException {
 }
diff -u b/core/lib/Drupal/Core/File/Exception/NonExistentFileException.php b/core/lib/Drupal/Core/File/Exception/NonExistentFileException.php
--- b/core/lib/Drupal/Core/File/Exception/NonExistentFileException.php
+++ b/core/lib/Drupal/Core/File/Exception/NonExistentFileException.php
@@ -1,11 +1,9 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\File\Exception\NonExistentFileException.
- */
-
 namespace Drupal\Core\File\Exception;
 
+/**
+ * Exception thrown when a file is expected to exist, but does not.
+ */
 class NonExistentFileException extends FileException {
 }
diff -u b/core/lib/Drupal/Core/File/UnmanagedFileHandler.php b/core/lib/Drupal/Core/File/UnmanagedFileHandler.php
--- b/core/lib/Drupal/Core/File/UnmanagedFileHandler.php
+++ b/core/lib/Drupal/Core/File/UnmanagedFileHandler.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Core\File\UnmanagedFileHandler.
- */
-
 namespace Drupal\Core\File;
 
 use Drupal\Core\File\Exception\DirectoryNotReadyException;
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/File/UnmanagedFileHandlerTest.php
@@ -0,0 +1,95 @@
+<?php
+
+namespace Drupal\Tests\Core\File;
+
+use Drupal\Core\File\Exception\NonExistentFileException;
+use Drupal\Core\File\FileHandlerInterface;
+use Drupal\Core\File\UnmanagedFileHandler;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\File\UnmanagedFileHandler
+ * @group File
+ */
+class UnmanagedFileHandlerTest extends KernelTestBase {
+
+  /**
+   * The file handler under test.
+   *
+   * @var \Drupal\Core\File\UnmanagedFileHandler
+   */
+  protected $fileHandler;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['system'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->fileHandler = new UnmanagedFileHandler(
+      $this->container->get('file_system')
+    );
+  }
+
+  /**
+   * @covers ::copy
+   */
+  public function testEnsureFileExistsBeforeCopy() {
+    // We need to compute the exception message here because it will include
+    // the 'real' path to the file, which varies with $this->siteDirectory.
+    $this->setExpectedException(
+      NonExistentFileException::class,
+      "File 'public://test.txt' ('{$this->siteDirectory}/files/test.txt') could not be copied because it does not exist"
+    );
+
+    $this->fileHandler->copy('public://test.txt', 'public://test-copy.txt');
+  }
+
+  /**
+   * @covers ::copy
+   * @expectedException \Drupal\Core\File\Exception\DirectoryNotReadyException
+   * @expectedExceptionMessage The specified file 'public://test.txt' could not be copied because the destination directory is not properly configured. This may be caused by a problem with file or directory permissions
+   */
+  public function testDestinationDirectoryFailureOnCopy() {
+    touch('public://test.txt');
+    // public://subdirectory has not been created, so file_prepare_directory()
+    // will fail, causing copy() to throw DirectoryNotReadyException.
+    $this->fileHandler->copy('public://test.txt', 'public://subdirectory/test.txt');
+  }
+
+  /**
+   * @covers ::copy
+   * @expectedException \Drupal\Core\File\Exception\FileExistsException
+   * @expectedExceptionMessage File 'public://test.txt' could not be copied because a file by that name already exists in the destination directory ('')
+   */
+  public function testCopyFailureIfFileAlreadyExists() {
+    $uri = 'public://test.txt';
+    touch($uri);
+    $this->fileHandler->copy($uri, $uri, FileHandlerInterface::FILE_EXISTS_ERROR);
+  }
+
+  /**
+   * @covers ::copy
+   * @expectedException \Drupal\Core\File\Exception\FileException
+   * @expectedException File 'public://test.txt' cannot be copied because it would overwrite itself
+   */
+  public function testCopyFailureIfSelfOverwrite() {
+    $uri = 'public://test.txt';
+    $this->fileHandler->copy($uri, $uri);
+  }
+
+  /**
+   * @covers ::copy
+   */
+  public function testSuccessfulCopy() {
+    touch('public://test.txt');
+    $this->fileHandler->copy('public://test.txt', 'public://test-copy.txt');
+    $this->assertFileExists('public://test-copy.txt');
+  }
+
+}