diff --git a/core/lib/Drupal/Component/Scaffold/AllowedPackages.php b/core/lib/Drupal/Component/Scaffold/AllowedPackages.php
index 5b0c25bb2d..edfc39bcc3 100644
--- a/core/lib/Drupal/Component/Scaffold/AllowedPackages.php
+++ b/core/lib/Drupal/Component/Scaffold/AllowedPackages.php
@@ -50,13 +50,13 @@ class AllowedPackages implements PostPackageEventListenerInterface {
    *   The composer object.
    * @param \Composer\IO\IOInterface $io
    *   IOInterface to write to.
-   * @param \Drupal\Component\Scaffold\ManageOptions $manageOptions
+   * @param \Drupal\Component\Scaffold\ManageOptions $manage_options
    *   Manager of the options in the top-level composer.json's 'extra' section.
    */
-  public function __construct(Composer $composer, IOInterface $io, ManageOptions $manageOptions) {
+  public function __construct(Composer $composer, IOInterface $io, ManageOptions $manage_options) {
     $this->composer = $composer;
     $this->io = $io;
-    $this->manageOptions = $manageOptions;
+    $this->manageOptions = $manage_options;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Scaffold/GenerateAutoloadReferenceFile.php b/core/lib/Drupal/Component/Scaffold/GenerateAutoloadReferenceFile.php
index a485ca3f49..64b240ec42 100644
--- a/core/lib/Drupal/Component/Scaffold/GenerateAutoloadReferenceFile.php
+++ b/core/lib/Drupal/Component/Scaffold/GenerateAutoloadReferenceFile.php
@@ -27,21 +27,21 @@ private function __construct() {
    *   The name of the package defining the autoload file (the root package).
    * @param string $web_root
    *   The path to the web root.
-   * @param string $vendorPath
+   * @param string $vendor
    *   The path to the vendor directory.
    *
    * @return \Drupal\Component\Scaffold\Operations\ScaffoldResult
    *   The result of the autoload file generation.
    */
-  public static function generateAutoload($package_name, $web_root, $vendorPath) {
-    $autoloadPath = static::autoloadPath($package_name, $web_root);
-    $location = dirname($autoloadPath->fullPath());
+  public static function generateAutoload($package_name, $web_root, $vendor) {
+    $autoload_path = static::autoloadPath($package_name, $web_root);
+    $location = dirname($autoload_path->fullPath());
     // Calculate the relative path from the webroot (location of the project
     // autoload.php) to the vendor directory.
     $fs = new SymfonyFilesystem();
-    $relativeVendorPath = $fs->makePathRelative($vendorPath, realpath($location));
-    $fs->dumpFile($autoloadPath->fullPath(), static::autoLoadContents($relativeVendorPath));
-    return new ScaffoldResult($autoloadPath, TRUE);
+    $relative_vendor_path = $fs->makePathRelative($vendor, realpath($location));
+    $fs->dumpFile($autoload_path->fullPath(), static::autoLoadContents($relative_vendor_path));
+    return new ScaffoldResult($autoload_path, TRUE);
   }
 
   /**
@@ -65,14 +65,14 @@ protected static function autoloadPath($package_name, $web_root) {
   /**
    * Builds the contents of the autoload file.
    *
-   * @param string $relativeVendorPath
+   * @param string $vendor_path
    *   The relative path to vendor.
    *
    * @return string
    *   Return the contents for the autoload.php.
    */
-  protected static function autoLoadContents($relativeVendorPath) {
-    $relativeVendorPath = rtrim($relativeVendorPath, '/');
+  protected static function autoLoadContents($vendor_path) {
+    $vendor_path = rtrim($vendor_path, '/');
     return <<<EOF
 <?php
 
@@ -89,7 +89,7 @@ protected static function autoLoadContents($relativeVendorPath) {
  * @see core/modules/statistics/statistics.php
  */
 
-return require __DIR__ . '/{$relativeVendorPath}/autoload.php';
+return require __DIR__ . '/{$vendor_path}/autoload.php';
 
 EOF;
   }
diff --git a/core/lib/Drupal/Component/Scaffold/Handler.php b/core/lib/Drupal/Component/Scaffold/Handler.php
index 161ce5a4f0..c0407e5f05 100644
--- a/core/lib/Drupal/Component/Scaffold/Handler.php
+++ b/core/lib/Drupal/Component/Scaffold/Handler.php
@@ -8,7 +8,6 @@
 use Composer\IO\IOInterface;
 use Composer\Package\PackageInterface;
 use Composer\Plugin\CommandEvent;
-use Composer\Script\Event;
 use Composer\Util\Filesystem;
 use Drupal\Component\Scaffold\Operations\OperationCollection;
 use Drupal\Component\Scaffold\Operations\OperationFactory;
diff --git a/core/lib/Drupal/Component/Scaffold/Interpolator.php b/core/lib/Drupal/Component/Scaffold/Interpolator.php
index 70577e26b5..a80ea26396 100644
--- a/core/lib/Drupal/Component/Scaffold/Interpolator.php
+++ b/core/lib/Drupal/Component/Scaffold/Interpolator.php
@@ -31,14 +31,14 @@ class Interpolator {
   /**
    * Interpolator constructor.
    *
-   * @param string $startToken
+   * @param string $start_token
    *   The start marker for a token, e.g. '['.
-   * @param string $endToken
+   * @param string $end_token
    *   The end marker for a token, e.g. ']'.
    */
-  public function __construct($startToken = '\\[', $endToken = '\\]') {
-    $this->startToken = $startToken;
-    $this->endToken = $endToken;
+  public function __construct($start_token = '\\[', $end_token = '\\]') {
+    $this->startToken = $start_token;
+    $this->endToken = $end_token;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Scaffold/ManageGitIgnore.php b/core/lib/Drupal/Component/Scaffold/ManageGitIgnore.php
index 28a37fb9f0..9985f3ca9f 100644
--- a/core/lib/Drupal/Component/Scaffold/ManageGitIgnore.php
+++ b/core/lib/Drupal/Component/Scaffold/ManageGitIgnore.php
@@ -29,7 +29,7 @@ public function __construct($dir) {
   /**
    * Manages gitignore files.
    *
-   * @param array $files
+   * @param \Drupal\Component\Scaffold\Operations\ScaffoldResult[] $files
    *   A list of scaffold results, each of which holds a path and whether
    *   or not that file is managed.
    * @param \Drupal\Component\Scaffold\ScaffoldOptions $options
diff --git a/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php b/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php
index c4488eac3f..d4d58b1b33 100644
--- a/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php
+++ b/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php
@@ -28,14 +28,14 @@ class AppendOp implements OperationInterface {
   /**
    * Constructs an AppendOp.
    *
-   * @param \Drupal\Component\Scaffold\ScaffoldFilePath $prependPath
+   * @param \Drupal\Component\Scaffold\ScaffoldFilePath $prepend_path
    *   The relative path to the prepend file.
-   * @param \Drupal\Component\Scaffold\ScaffoldFilePath $appendPath
+   * @param \Drupal\Component\Scaffold\ScaffoldFilePath $append_path
    *   The relative path to the append file.
    */
-  public function __construct(ScaffoldFilePath $prependPath = NULL, ScaffoldFilePath $appendPath = NULL) {
-    $this->prepend = $prependPath;
-    $this->append = $appendPath;
+  public function __construct(ScaffoldFilePath $prepend_path = NULL, ScaffoldFilePath $append_path = NULL) {
+    $this->prepend = $prepend_path;
+    $this->append = $append_path;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php b/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php
index 1e513b1e27..48ffdf8ee8 100644
--- a/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php
+++ b/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php
@@ -40,7 +40,7 @@ public function __construct(IOInterface $io) {
    * this will be the same as $scaffold_file->packageName() unless this scaffold
    * file has been overridden or removed by some other package.
    *
-   * @param array $list_of_scaffold_files
+   * @param \Drupal\Component\Scaffold\ScaffoldFileInfo[] $list_of_scaffold_files
    *   Associative array containing destination => operation mappings.
    * @param \Drupal\Component\Scaffold\ScaffoldFileInfo $scaffold_file
    *   The scaffold file to use to find a providing package name.
@@ -55,16 +55,16 @@ protected function findProvidingPackage(array $list_of_scaffold_files, ScaffoldF
     if (!array_key_exists($dest_rel_path, $list_of_scaffold_files)) {
       throw new \RuntimeException("Scaffold file not found in list of all scaffold files.");
     }
-    $overridden_scaffold_file = $list_of_scaffold_files[$dest_rel_path];
-    return $overridden_scaffold_file->packageName();
+    return $list_of_scaffold_files[$dest_rel_path]->packageName();
   }
 
   /**
    * Process all of the scaffold files listed in the provided file mappings.
+   *
    * @param array $file_mappings
    *   An multidimensional array of file mappings, as returned by
    *   self::getFileMappingsFromPackages().
-   * @param \Drupal\Component\Scaffold\Interpolator $locationReplacements
+   * @param \Drupal\Component\Scaffold\Interpolator $location_replacements
    *   An object with the location mappings (e.g. [web-root]).
    * @param \Drupal\Component\Scaffold\ScaffoldOptions $options
    *   Configuration options from the top-level composer.json file.
@@ -73,8 +73,8 @@ protected function findProvidingPackage(array $list_of_scaffold_files, ScaffoldF
    *   Associative array keyed by destination path and values as the scaffold
    *   result for each scaffolded file.
    */
-  public function process(array $file_mappings, Interpolator $locationReplacements, ScaffoldOptions $options) {
-    list($list_of_scaffold_files, $resolved_file_mappings) = $this->collateScaffoldFiles($file_mappings, $locationReplacements);
+  public function process(array $file_mappings, Interpolator $location_replacements, ScaffoldOptions $options) {
+    list($list_of_scaffold_files, $resolved_file_mappings) = $this->collateScaffoldFiles($file_mappings, $location_replacements);
     return $this->processScaffoldFiles($list_of_scaffold_files, $resolved_file_mappings, $options);
   }
 
@@ -84,7 +84,7 @@ public function process(array $file_mappings, Interpolator $locationReplacements
    * @param array $file_mappings
    *   An multidimensional array of file mappings, as returned by
    *   self::getFileMappingsFromPackages().
-   * @param \Drupal\Component\Scaffold\Interpolator $locationReplacements
+   * @param \Drupal\Component\Scaffold\Interpolator $location_replacements
    *   An object with the location mappings (e.g. [web-root]).
    *
    * @return array
@@ -92,14 +92,16 @@ public function process(array $file_mappings, Interpolator $locationReplacements
    *     - Associative array containing destination => operation mappings.
    *     - Associative array containing package name => file mappings.
    */
-  protected function collateScaffoldFiles(array $file_mappings, Interpolator $locationReplacements) {
+  protected function collateScaffoldFiles(array $file_mappings, Interpolator $location_replacements) {
     $resolved_file_mappings = [];
+    /** @var \Drupal\Component\Scaffold\ScaffoldFileInfo[] $list_of_scaffold_files */
     $list_of_scaffold_files = [];
     foreach ($file_mappings as $package_name => $package_file_mappings) {
       foreach ($package_file_mappings as $destination_rel_path => $op) {
-        $destination = ScaffoldFilePath::destinationPath($package_name, $destination_rel_path, $locationReplacements);
+        $destination = ScaffoldFilePath::destinationPath($package_name, $destination_rel_path, $location_replacements);
         // If there was already a scaffolding operation happening at this path,
-        // use a ConjunctionOp if possible.
+        // AppendOp operations can be joined to the previous operation using a
+        // ConjunctionOp.
         if (isset($list_of_scaffold_files[$destination_rel_path]) && $op instanceof AppendOp) {
           $op = new ConjunctionOp($list_of_scaffold_files[$destination_rel_path]->op(), $op);
         }
diff --git a/core/lib/Drupal/Component/Scaffold/Operations/OperationFactory.php b/core/lib/Drupal/Component/Scaffold/Operations/OperationFactory.php
index ad6f531837..7ede3241d6 100644
--- a/core/lib/Drupal/Component/Scaffold/Operations/OperationFactory.php
+++ b/core/lib/Drupal/Component/Scaffold/Operations/OperationFactory.php
@@ -34,7 +34,7 @@ public function __construct(Composer $composer) {
    *
    * @param \Composer\Package\PackageInterface $package
    *   The package that relative paths will be relative from.
-   * @param string $dest_rel_path
+   * @param string $destination
    *   The destination path for the scaffold file. Used only for error messages.
    * @param mixed $metadata
    *   The metadata for this operation object, which varies by operation type.
@@ -46,17 +46,17 @@ public function __construct(Composer $composer) {
    *   Exception thrown when $metadata can not be used to determine a scaffold
    *   operation.
    */
-  public function create(PackageInterface $package, $dest_rel_path, $metadata) {
-    $metadata = $this->normalizeScaffoldMetadata($dest_rel_path, $metadata);
+  public function create(PackageInterface $package, $destination, $metadata) {
+    $metadata = $this->normalizeScaffoldMetadata($destination, $metadata);
     switch ($metadata['mode']) {
       case 'skip':
         return new SkipOp();
 
       case 'replace':
-        return $this->createReplaceOp($package, $dest_rel_path, $metadata);
+        return $this->createReplaceOp($package, $destination, $metadata);
 
       case 'append':
-        return $this->createAppendOp($package, $dest_rel_path, $metadata);
+        return $this->createAppendOp($package, $destination, $metadata);
     }
     throw new \RuntimeException("Unknown scaffold operation mode <comment>{$metadata['mode']}</comment>.");
   }
@@ -68,7 +68,7 @@ public function create(PackageInterface $package, $dest_rel_path, $metadata) {
    *
    * @param \Composer\Package\PackageInterface $package
    *   The package that relative paths will be relative from.
-   * @param string $dest_rel_path
+   * @param string $destination
    *   The destination path for the scaffold file. Used only for error messages.
    * @param array $metadata
    *   The metadata for this operation object, i.e. the relative 'path'.
@@ -76,7 +76,7 @@ public function create(PackageInterface $package, $dest_rel_path, $metadata) {
    * @return \Drupal\Component\Scaffold\Operations\OperationInterface
    *   A scaffold replace operation object.
    */
-  protected function createReplaceOp(PackageInterface $package, $dest_rel_path, array $metadata) {
+  protected function createReplaceOp(PackageInterface $package, $destination, array $metadata) {
     // If this op does not provide an 'overwrite' value, default it to true.
     $metadata += ['overwrite' => TRUE];
     if (!isset($metadata['path'])) {
@@ -84,7 +84,7 @@ protected function createReplaceOp(PackageInterface $package, $dest_rel_path, ar
     }
     $package_name = $package->getName();
     $package_path = $this->getPackagePath($package);
-    $source = ScaffoldFilePath::sourcePath($package_name, $package_path, $dest_rel_path, $metadata['path']);
+    $source = ScaffoldFilePath::sourcePath($package_name, $package_path, $destination, $metadata['path']);
     $op = new ReplaceOp($source, $metadata['overwrite']);
     return $op;
   }
@@ -94,7 +94,7 @@ protected function createReplaceOp(PackageInterface $package, $dest_rel_path, ar
    *
    * @param \Composer\Package\PackageInterface $package
    *   The package that relative paths will be relative from.
-   * @param string $dest_rel_path
+   * @param string $destination
    *   The destination path for the scaffold file. Used only for error messages.
    * @param array $metadata
    *   The metadata for this operation object, i.e. the relative 'path'.
@@ -102,16 +102,16 @@ protected function createReplaceOp(PackageInterface $package, $dest_rel_path, ar
    * @return \Drupal\Component\Scaffold\Operations\OperationInterface
    *   A scaffold replace operation object.
    */
-  protected function createAppendOp(PackageInterface $package, $dest_rel_path, array $metadata) {
+  protected function createAppendOp(PackageInterface $package, $destination, array $metadata) {
     $package_name = $package->getName();
     $package_path = $this->getPackagePath($package);
     $prepend_source_file = NULL;
     $append_source_file = NULL;
     if (isset($metadata['prepend'])) {
-      $prepend_source_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $dest_rel_path, $metadata['prepend']);
+      $prepend_source_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $destination, $metadata['prepend']);
     }
     if (isset($metadata['append'])) {
-      $append_source_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $dest_rel_path, $metadata['append']);
+      $append_source_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $destination, $metadata['append']);
     }
     $op = new AppendOp($prepend_source_file, $append_source_file);
     return $op;
@@ -149,7 +149,7 @@ protected function getPackagePath(PackageInterface $package) {
    *   - Boolean 'false' means "skip".
    *   - A string means "replace", with the string value becoming the path.
    *
-   * @param string $dest_rel_path
+   * @param string $destination
    *   The destination path for the scaffold file.
    * @param mixed $value
    *   The metadata for this operation object, which varies by operation type.
@@ -157,15 +157,15 @@ protected function getPackagePath(PackageInterface $package) {
    * @return array
    *   Normalized scaffold metadata.
    */
-  protected function normalizeScaffoldMetadata($dest_rel_path, $value) {
+  protected function normalizeScaffoldMetadata($destination, $value) {
     if (is_bool($value)) {
       if (!$value) {
         return ['mode' => 'skip'];
       }
-      throw new \RuntimeException("File mapping {$dest_rel_path} cannot be given the value 'true'.");
+      throw new \RuntimeException("File mapping {$destination} cannot be given the value 'true'.");
     }
     if (empty($value)) {
-      throw new \RuntimeException("File mapping {$dest_rel_path} cannot be empty.");
+      throw new \RuntimeException("File mapping {$destination} cannot be empty.");
     }
     if (is_string($value)) {
       $value = ['path' => $value];
diff --git a/core/lib/Drupal/Component/Scaffold/Operations/ReplaceOp.php b/core/lib/Drupal/Component/Scaffold/Operations/ReplaceOp.php
index a980e9d2bf..358f2cdc5f 100644
--- a/core/lib/Drupal/Component/Scaffold/Operations/ReplaceOp.php
+++ b/core/lib/Drupal/Component/Scaffold/Operations/ReplaceOp.php
@@ -40,26 +40,6 @@ public function __construct(ScaffoldFilePath $sourcePath, $overwrite = TRUE) {
     $this->overwrite = $overwrite;
   }
 
-  /**
-   * Gets the source.
-   *
-   * @return \Drupal\Component\Scaffold\ScaffoldFilePath
-   *   The source file reference object.
-   */
-  protected function getSource() {
-    return $this->source;
-  }
-
-  /**
-   * Determines whether scaffold file should overwrite files.
-   *
-   * @return bool
-   *   Value of the 'overwrite' option.
-   */
-  protected function getOverwrite() {
-    return $this->overwrite;
-  }
-
   /**
    * Copy or Symlink the specified scaffold file.
    *
@@ -70,7 +50,7 @@ public function process(ScaffoldFilePath $destination, IOInterface $io, Scaffold
     $destination_path = $destination->fullPath();
     // Do nothing if overwrite is 'false' and a file already exists at the
     // destination.
-    if ($this->getOverwrite() === FALSE && file_exists($destination_path)) {
+    if ($this->overwrite === FALSE && file_exists($destination_path)) {
       $interpolator = $destination->getInterpolator();
       $io->write($interpolator->interpolate("  - Skip <info>[dest-rel-path]</info> because it already exists and overwrite is <comment>false</comment>."));
       return new ScaffoldResult($destination, FALSE);
@@ -99,13 +79,13 @@ public function process(ScaffoldFilePath $destination, IOInterface $io, Scaffold
    */
   protected function copyScaffold(ScaffoldFilePath $destination, IOInterface $io) {
     $interpolator = $destination->getInterpolator();
-    $this->getSource()->addInterpolationData($interpolator);
-    $success = copy($this->getSource()->fullPath(), $destination->fullPath());
+    $this->source->addInterpolationData($interpolator);
+    $success = copy($this->source->fullPath(), $destination->fullPath());
     if (!$success) {
       throw new \RuntimeException($interpolator->interpolate("Could not copy source file <info>[src-rel-path]</info> to <info>[dest-rel-path]</info>!"));
     }
     $io->write($interpolator->interpolate("  - Copy <info>[dest-rel-path]</info> from <info>[src-rel-path]</info>"));
-    return new ScaffoldResult($destination, $this->getOverwrite());
+    return new ScaffoldResult($destination, $this->overwrite);
   }
 
   /**
@@ -123,13 +103,13 @@ protected function symlinkScaffold(ScaffoldFilePath $destination, IOInterface $i
     $interpolator = $destination->getInterpolator();
     try {
       $fs = new Filesystem();
-      $fs->relativeSymlink($this->getSource()->fullPath(), $destination->fullPath());
+      $fs->relativeSymlink($this->source->fullPath(), $destination->fullPath());
     }
     catch (\Exception $e) {
       throw new \RuntimeException($interpolator->interpolate("Could not symlink source file <info>[src-rel-path]</info> to <info>[dest-rel-path]</info>!"), [], $e);
     }
     $io->write($interpolator->interpolate("  - Link <info>[dest-rel-path]</info> from <info>[src-rel-path]</info>"));
-    return new ScaffoldResult($destination, $this->getOverwrite());
+    return new ScaffoldResult($destination, $this->overwrite);
   }
 
 }
diff --git a/core/lib/Drupal/Component/Scaffold/ScaffoldFilePath.php b/core/lib/Drupal/Component/Scaffold/ScaffoldFilePath.php
index 35a1428fbe..8e842a68e1 100644
--- a/core/lib/Drupal/Component/Scaffold/ScaffoldFilePath.php
+++ b/core/lib/Drupal/Component/Scaffold/ScaffoldFilePath.php
@@ -35,7 +35,7 @@ class ScaffoldFilePath {
    *
    * @var string
    */
-  protected $relPath;
+  protected $relativePath;
 
   /**
    * The full path to the file.
@@ -59,7 +59,7 @@ class ScaffoldFilePath {
   public function __construct($path_type, $package_name, $rel_path, $full_path) {
     $this->type = $path_type;
     $this->packageName = $package_name;
-    $this->relPath = $rel_path;
+    $this->relativePath = $rel_path;
     $this->fullPath = $full_path;
   }
 
@@ -80,7 +80,7 @@ public function packageName() {
    *   Relative path to file.
    */
   public function relativePath() {
-    return $this->relPath;
+    return $this->relativePath;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Scaffold/ScaffoldOptions.php b/core/lib/Drupal/Component/Scaffold/ScaffoldOptions.php
index 91752c3660..ddc86659a0 100644
--- a/core/lib/Drupal/Component/Scaffold/ScaffoldOptions.php
+++ b/core/lib/Drupal/Component/Scaffold/ScaffoldOptions.php
@@ -60,16 +60,6 @@ public static function create(array $extras) {
     return new self($options);
   }
 
-  /**
-   * Creates a scaffold option object with default values.
-   *
-   * @return self
-   *   A scaffold options object with default values
-   */
-  public static function defaultOptions() {
-    return new self([]);
-  }
-
   /**
    * Creates a new scaffold options object with some values overridden.
    *
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/ExecTrait.php b/core/tests/Drupal/Tests/Component/Scaffold/ExecTrait.php
index 14c44ee6f4..0986961788 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/ExecTrait.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/ExecTrait.php
@@ -28,7 +28,7 @@ protected function mustExec($cmd, $cwd, array $env = []) {
     $process->setTimeout(300)->setIdleTimeout(300)->run();
     $exitCode = $process->getExitCode();
     if (0 != $exitCode) {
-      throw new \Exception("Exit code: {$exitCode}\n\n" . $process->getErrorOutput() . "\n\n" . $process->getOutput());
+      throw new \RuntimeException("Exit code: {$exitCode}\n\n" . $process->getErrorOutput() . "\n\n" . $process->getOutput());
     }
     return [$process->getOutput(), $process->getErrorOutput()];
   }
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php b/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php
index 86dab883e1..de91e9381b 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\Component\Scaffold;
 
-use Composer\Composer;
 use Composer\Console\Application;
 use Composer\Factory;
 use Composer\IO\BufferIO;
@@ -19,20 +18,34 @@
  * Convenience class for creating fixtures.
  */
 class Fixtures {
+
   /**
    * Directories to delete when we are done.
    *
    * @var string[]
    */
   protected $tmpDirs = [];
+
+  /**
+   * A Composer IOInterface to write to.
+   *
+   * @var \Composer\IO\IOInterface
+   */
   protected $io;
+
+  /**
+   * The composer object.
+   *
+   * @var \Composer\Composer
+   */
   protected $composer;
 
   /**
-   * Get an IO fixture.
+   * Gets an IO fixture.
    *
    * @return \Composer\IO\IOInterface
-   *   A Composer IOInterface to write to; output may be retrieved via Fixtures::getOutput()
+   *   A Composer IOInterface to write to; output may be retrieved via
+   *   Fixtures::getOutput().
    */
   public function io() {
     if (!$this->io) {
@@ -42,7 +55,7 @@ public function io() {
   }
 
   /**
-   * Get the Composer object.
+   * Gets the Composer object.
    *
    * @return \Composer\Composer
    *   The main Composer object, needed by the scaffold Handler, etc.
@@ -55,7 +68,7 @@ public function getComposer() {
   }
 
   /**
-   * Get the output from our io() fixture.
+   * Gets the output from the io() fixture.
    *
    * @return string
    *   Output captured from tests that write to Fixtures::io().
@@ -65,7 +78,9 @@ public function getOutput() {
   }
 
   /**
-   * Return the path to this project so that it may be injected into composer.json files.
+   * Gets the path to Scaffold component.
+   *
+   * Used to inject the component into composer.json files.
    *
    * @return string
    *   Path to the root of this project.
@@ -75,7 +90,7 @@ public function projectRoot() {
   }
 
   /**
-   * Return the path to the project fixtures.
+   * Gets the path to the project fixtures.
    *
    * @return string
    *   Path to project fixtures
@@ -85,59 +100,71 @@ public function allFixturesDir() {
   }
 
   /**
-   * Return the path to one particular project fixture.
+   * Gets the path to one particular project fixture.
+   *
+   * @param string $project_name
+   *   The project name to get the path for.
    *
    * @return string
-   *   Path to project fixture
+   *   Path to project fixture.
    */
   public function projectFixtureDir($project_name) {
     $dir = $this->allFixturesDir() . '/' . $project_name;
     if (!is_dir($dir)) {
-      throw new \Exception("Requested fixture project {$project_name} that does not exist.");
+      throw new \RuntimeException("Requested fixture project {$project_name} that does not exist.");
     }
     return $dir;
   }
 
   /**
-   * Return the path to one particular bin path.
+   * Gets the path to one particular bin path.
+   *
+   * @param string $bin_name
+   *   The bin name to get the path for.
    *
    * @return string
-   *   Path to project fixture
+   *   Path to project fixture.
    */
   public function binFixtureDir($bin_name) {
     $dir = $this->allFixturesDir() . '/scripts/' . $bin_name;
     if (!is_dir($dir)) {
-      throw new \Exception("Requested fixture bin dir {$bin_name} that does not exist.");
+      throw new \RuntimeException("Requested fixture bin dir {$bin_name} that does not exist.");
     }
     return $dir;
   }
 
   /**
-   * Use in place of ScaffoldFilePath::sourcePath to get a path to a source scaffold fixture.
+   * Gets a path to a source scaffold fixture.
+   *
+   * Use in place of ScaffoldFilePath::sourcePath().
    *
    * @param string $project_name
-   *   The name of the project to fetch; $package_name is "fixtures/$project_name".
+   *   The name of the project to fetch; $package_name is
+   *   "fixtures/$project_name".
    * @param string $source
    *   The name of the asset; path is "assets/$source".
-   * @param string $destination
-   *   The path to the destination; only used in error messages, not needed for most tests.
    *
    * @return \Drupal\Component\Scaffold\ScaffoldFilePath
    *   The full and relative path to the desired asset
+   *
+   * @see \Drupal\Component\Scaffold\ScaffoldFilePath::sourcePath()
    */
-  public function sourcePath($project_name, $source, $destination = 'unknown') {
+  public function sourcePath($project_name, $source) {
     $package_name = "fixtures/{$project_name}";
     $source_rel_path = "assets/{$source}";
     $package_path = $this->projectFixtureDir($project_name);
-    $destination = 'unknown';
-    return ScaffoldFilePath::sourcePath($package_name, $package_path, $destination, $source_rel_path);
+    return ScaffoldFilePath::sourcePath($package_name, $package_path, 'unknown', $source_rel_path);
   }
 
   /**
-   * Use in place of Handler::getLocationReplacements() to obtain a 'web-root'.
+   * Gets an Interpolator with 'web-root' and 'package-name' set.
+   *
+   * Use in place of ManageOptions::getLocationReplacements().
    *
    * @return \Drupal\Component\Scaffold\Interpolator
    *   An interpolator with location replacements, including 'web-root'.
+   *
+   * @see \Drupal\Component\Scaffold\ManageOptions::getLocationReplacements()
    */
   public function getLocationReplacements() {
     $destinationTmpDir = $this->mkTmpDir();
@@ -147,10 +174,11 @@ public function getLocationReplacements() {
   }
 
   /**
-   * Use to create a ReplaceOp fixture.
+   * Creates a ReplaceOp fixture.
    *
    * @param string $project_name
-   *   The name of the project to fetch; $package_name is "fixtures/$project_name".
+   *   The name of the project to fetch; $package_name is
+   *   "fixtures/$project_name".
    * @param string $source
    *   The name of the asset; path is "assets/$source".
    *
@@ -163,15 +191,16 @@ public function replaceOp($project_name, $source) {
   }
 
   /**
-   * Use to create an AppendOp fixture.
+   * Creates an AppendOp fixture.
    *
    * @param string $project_name
-   *   The name of the project to fetch; $package_name is "fixtures/$project_name".
+   *   The name of the project to fetch; $package_name is
+   *   "fixtures/$project_name".
    * @param string $source
    *   The name of the asset; path is "assets/$source".
    *
    * @return \Drupal\Component\Scaffold\Operations\AppendOp
-   *   An append opperation object.
+   *   An append operation object.
    */
   public function appendOp($project_name, $source) {
     $source_path = $this->sourcePath($project_name, $source);
@@ -179,7 +208,9 @@ public function appendOp($project_name, $source) {
   }
 
   /**
-   * Use in place of ScaffoldFilePath::destinationPath to get a destination path in a tmp dir.
+   * Gets a destination path in a tmp dir.
+   *
+   * Use in place of ScaffoldFilePath::destinationPath().
    *
    * @param string $destination
    *   Destination path; should be in the form '[web-root]/robots.txt', where
@@ -189,11 +220,13 @@ public function appendOp($project_name, $source) {
    *   Location replacements. Obtain via Fixtures::getLocationReplacements()
    *   when creating multiple scaffold destinations.
    * @param string $package_name
-   *   The name of the fixture package that this path came from. Optional;
-   *   taken from interpolator if not provided.
+   *   (optional) The name of the fixture package that this path came from.
+   *   Taken from interpolator if not provided.
    *
    * @return \Drupal\Component\Scaffold\ScaffoldFilePath
    *   A destination scaffold file backed by temporary storage.
+   *
+   * @see \Drupal\Component\Scaffold\ScaffoldFilePath::destinationPath()
    */
   public function destinationPath($destination, Interpolator $interpolator = NULL, $package_name = NULL) {
     $interpolator = $interpolator ?: $this->getLocationReplacements();
@@ -202,7 +235,7 @@ public function destinationPath($destination, Interpolator $interpolator = NULL,
   }
 
   /**
-   * Generate a path to a temporary location, but do not create the directory.
+   * Generates a path to a temporary location, but do not create the directory.
    *
    * @param string $extraSalt
    *   Extra characters to throw into the md5 to add to name.
@@ -217,7 +250,7 @@ public function tmpDir($extraSalt = '') {
   }
 
   /**
-   * Create a temporary directory.
+   * Creates a temporary directory.
    *
    * @param string $extraSalt
    *   Extra characters to throw into the md5 to add to name.
@@ -233,7 +266,7 @@ public function mkTmpDir($extraSalt = '') {
   }
 
   /**
-   * Call 'tearDown' in any test that copies fixtures to transient locations.
+   * Calls 'tearDown' in any test that copies fixtures to transient locations.
    */
   public function tearDown() {
     // Remove any temporary directories that were created.
@@ -243,28 +276,28 @@ public function tearDown() {
     }
     // Clear out variables from the previous pass.
     $this->tmpDirs = [];
-    $this->fixturesDir = NULL;
     $this->io = NULL;
   }
 
   /**
-   * Create a temporary copy of all of the fixtures projects into a temp dir.
+   * Creates a temporary copy of all of the fixtures projects into a temp dir.
    *
    * The fixtures remain dirty if they already exist. Individual tests should
-   * first delete any fixture directory that needs to remain pristine. Since
-   * all temporary directories are removed in tearDown, this is only an issue
-   * when a) the FIXTURE_DIR environment variable has been set, or b) tests
-   * are calling cloneFixtureProjects more than once per test method.
+   * first delete any fixture directory that needs to remain pristine. Since all
+   * temporary directories are removed in tearDown, this is only an issue when
+   * a) the FIXTURE_DIR environment variable has been set, or b) tests are
+   * calling cloneFixtureProjects more than once per test method.
    *
    * @param string $fixturesDir
    *   The directory to place fixtures in.
    * @param array $replacements
-   *   Key : value mappings for placeholders to replace in composer.json templates.
+   *   Key : value mappings for placeholders to replace in composer.json
+   *   templates.
    */
   public function cloneFixtureProjects($fixturesDir, array $replacements = []) {
     $filesystem = new Filesystem();
     $replacements += ['SYMLINK' => 'true'];
-    $interpolator = new Interpolator('__', '__', TRUE);
+    $interpolator = new Interpolator('__', '__');
     $interpolator->setData($replacements);
     $filesystem->copy($this->allFixturesDir(), $fixturesDir);
     $composer_json_templates = glob($fixturesDir . "/*/composer.json.tmpl");
@@ -280,12 +313,18 @@ public function cloneFixtureProjects($fixturesDir, array $replacements = []) {
   }
 
   /**
-   * Run the scaffold operation.
+   * Runs the scaffold operation.
+   *
+   * This is equivalent to running `composer composer-scaffold`, but we do the
+   * equivalent operation by instantiating a Handler object in order to continue
+   * running in the same process, so that coverage may be calculated for the
+   * code executed by these tests.
    *
-   * This is equivalent to running `composer composer-scaffold`, but we
-   * do the equivalent operation by instantiating a Handler object in order
-   * to continue running in the same process, so that coverage may be
-   * calculated for the code executed by these tests.
+   * @param string $cwd
+   *   The working directory to run the scaffold command in.
+   *
+   * @return string
+   *   Output captured from tests that write to Fixtures::io().
    */
   public function runScaffold($cwd) {
     chdir($cwd);
@@ -304,8 +343,8 @@ public function runScaffold($cwd) {
    * @param int $expectedExitCode
    *   The expected exit code; will throw if a different exit code is returned.
    *
-   * @return array
-   *   Standard output and standard error from the command
+   * @return string
+   *   Standard output and standard error from the command.
    */
   public function runComposer($cmd, $cwd, $expectedExitCode = 0) {
     chdir($cwd);
@@ -315,13 +354,13 @@ public function runComposer($cmd, $cwd, $expectedExitCode = 0) {
     $application->setAutoExit(FALSE);
     try {
       $exitCode = $application->run($input, $output);
+      if ($exitCode != $expectedExitCode) {
+        print "Command '{$cmd}' - Expected exit code: {$expectedExitCode}, actual exit code: {$exitCode}\n";
+      }
     }
     catch (\Exception $e) {
       print "Exception: " . $e->getMessage() . "\n";
     }
-    if ($exitCode != $expectedExitCode) {
-      print "Command '{$cmd}' - Expected exit code: {$expectedExitCode}, actual exit code: {$exitCode}\n";
-    }
     $output = $output->fetch();
     return $output;
   }
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ComposerHookTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ComposerHookTest.php
index 12fd9f1166..c48d8688c7 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ComposerHookTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ComposerHookTest.php
@@ -25,6 +25,7 @@
 class ComposerHookTest extends TestCase {
   use ExecTrait;
   use AssertUtilsTrait;
+
   /**
    * The root of this project.
    *
@@ -34,12 +35,14 @@ class ComposerHookTest extends TestCase {
    * @var string
    */
   protected $projectRoot;
+
   /**
    * Directory to perform the tests in.
    *
    * @var string
    */
   protected $fixturesDir;
+
   /**
    * The Symfony FileSystem component.
    *
@@ -47,6 +50,13 @@ class ComposerHookTest extends TestCase {
    */
   protected $fileSystem;
 
+  /**
+   * The Fixtures object.
+   *
+   * @var \Drupal\Tests\Component\Scaffold\Fixtures
+   */
+  protected $fixtures;
+
   /**
    * {@inheritdoc}
    */
@@ -82,7 +92,7 @@ public function testComposerHooks() {
     // project is "allowed" in our main fixture project, but not required.
     // We expect that requiring this library should re-scaffold, resulting
     // in a changed default.settings.php file.
-    list($stdout, $stderr) = $this->execComposer("require --no-ansi --no-interaction fixtures/scaffold-override-fixture:dev-master", $sut);
+    list($stdout,) = $this->execComposer("require --no-ansi --no-interaction fixtures/scaffold-override-fixture:dev-master", $sut);
     $this->assertScaffoldedFile($sut . '/sites/default/default.settings.php', $is_link, '#scaffolded from the scaffold-override-fixture#');
     // Make sure that the appropriate notice informing us that scaffolding
     // is allowed was printed.
@@ -99,11 +109,10 @@ public function testComposerHooks() {
     $this->assertScaffoldedFile($sut . '/sites/default/default.settings.php', $is_link, '#scaffolded from the scaffold-override-fixture#');
     // Run 'composer create-project' to create a new test project called
     // 'create-project-test', which is a copy of 'fixtures/drupal-drupal'.
-    $packages = $this->fixturesDir . '/packages.json';
     $sut = $this->fixturesDir . '/create-project-test';
     $filesystem = new Filesystem();
     $filesystem->remove($sut);
-    list($stdout, $stderr) = $this->execComposer("create-project --repository=packages.json fixtures/drupal-drupal {$sut}", $this->fixturesDir, ['COMPOSER_MIRROR_PATH_REPOS' => 1]);
+    list($stdout,) = $this->execComposer("create-project --repository=packages.json fixtures/drupal-drupal {$sut}", $this->fixturesDir, ['COMPOSER_MIRROR_PATH_REPOS' => 1]);
     $this->assertDirectoryExists($sut);
     $this->assertContains('Scaffolding files for fixtures/drupal-drupal', $stdout);
     $this->assertScaffoldedFile($sut . '/index.php', FALSE, '#Test version of index.php from drupal/core#');
@@ -113,7 +122,7 @@ public function testComposerHooks() {
     $this->execComposer("install --no-ansi", $sut);
     // Require a project that is not allowed to scaffold and confirm that we
     // get a warning, and it does not scaffold.
-    list($stdout, $stderr) = $this->execComposer("require --no-ansi --no-interaction fixtures/scaffold-override-fixture:dev-master", $sut);
+    list($stdout,) = $this->execComposer("require --no-ansi --no-interaction fixtures/scaffold-override-fixture:dev-master", $sut);
     $this->assertFileNotExists($sut . '/sites/default/default.settings.php');
     $this->assertContains('Package fixtures/scaffold-override-fixture has scaffold operations, but it is not allowed in the root-level composer.json file.', $stdout);
   }
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php
index 149063c58a..fc6f63f8c7 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php
@@ -20,6 +20,7 @@
 class ManageGitIgnoreTest extends TestCase {
   use ExecTrait;
   use AssertUtilsTrait;
+
   /**
    * The root of this project.
    *
@@ -29,12 +30,14 @@ class ManageGitIgnoreTest extends TestCase {
    * @var string
    */
   protected $projectRoot;
+
   /**
    * Directory to perform the tests in.
    *
    * @var string
    */
   protected $fixturesDir;
+
   /**
    * The Symfony FileSystem component.
    *
@@ -42,6 +45,13 @@ class ManageGitIgnoreTest extends TestCase {
    */
   protected $fileSystem;
 
+  /**
+   * The Fixtures object.
+   *
+   * @var \Drupal\Tests\Component\Scaffold\Fixtures
+   */
+  protected $fixtures;
+
   /**
    * {@inheritdoc}
    */
@@ -60,12 +70,19 @@ protected function tearDown() {
   }
 
   /**
-   * Create a system-under-test and initialize a git repository for it.
+   * Creates a system-under-test and initialize a git repository for it.
+   *
+   * @param string $fixture_name
+   *   The name of the fixture to use from
+   *   core/tests/Drupal/Tests/Component/Scaffold/fixtures.
+   *
+   * @return string
+   *   The path to the fixture directory.
    */
-  protected function createSutWithGit($topLevelProjectDir) {
+  protected function createSutWithGit($fixture_name) {
     $is_link = FALSE;
     $this->fixturesDir = $this->fixtures->tmpDir($this->getName());
-    $sut = $this->fixturesDir . '/' . $topLevelProjectDir;
+    $sut = $this->fixturesDir . '/' . $fixture_name;
     $replacements = ['SYMLINK' => $is_link ? 'true' : 'false', 'PROJECT_ROOT' => $this->projectRoot];
     $this->fixtures->cloneFixtureProjects($this->fixturesDir, $replacements);
     // .gitignore files will not be managed unless there is a git repository.
@@ -75,13 +92,13 @@ protected function createSutWithGit($topLevelProjectDir) {
     $this->mustExec('git config user.name "Test User"', $sut);
     $this->mustExec('git add .', $sut);
     $this->mustExec('git commit -m "Initial commit."', $sut);
-    // Run composer install, but supress scaffolding.
+    // Run composer install, but suppress scaffolding.
     $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     return $sut;
   }
 
   /**
-   * Test to see if the scaffold operation correctly manages the .gitignore file.
+   * Tests scaffold command correctly manages the .gitignore file.
    */
   public function testManageGitIgnore() {
     // Note that the drupal-composer-drupal-project fixture does not
@@ -106,9 +123,9 @@ public function testManageGitIgnore() {
 update.php
 web.config
 EOT;
-    // At this point we should have a .gitignore file, because although we
-    // did not explicitly ask for .gitignore tracking, the vendor directory
-    // is not tracked, so the default in that instance is to manage .gitignore files.
+    // At this point we should have a .gitignore file, because although we did
+    // not explicitly ask for .gitignore tracking, the vendor directory is not
+    // tracked, so the default in that instance is to manage .gitignore files.
     $this->assertScaffoldedFile($sut . '/docroot/.gitignore', FALSE, '#' . $expected . '#msi');
     $this->assertScaffoldedFile($sut . '/docroot/sites/.gitignore', FALSE, '#example.settings.local.php#');
     $this->assertScaffoldedFile($sut . '/docroot/sites/default/.gitignore', FALSE, '#default.services.yml#');
@@ -119,12 +136,12 @@ public function testManageGitIgnore() {
 EOT;
     // Check to see whether there are any untracked files. We expect that
     // only the .gitignore files themselves should be untracked.
-    list($stdout, $stderr) = $this->mustExec('git status --porcelain', $sut);
+    list($stdout,) = $this->mustExec('git status --porcelain', $sut);
     $this->assertEquals(trim($expected), trim($stdout));
   }
 
   /**
-   * Test to see if scaffold operation does not manage the .gitignore file when disabled.
+   * Tests scaffold command does not manage the .gitignore file when disabled.
    */
   public function testUnmanagedGitIgnoreWhenDisabled() {
     // Note that the drupal-drupal fixture has a configuration setting
@@ -139,24 +156,26 @@ public function testUnmanagedGitIgnoreWhenDisabled() {
   }
 
   /**
-   * Test to see if the scaffold operation disables .gitignore management when git not present.
+   * Tests scaffold command disables .gitignore management when git not present.
    *
-   * The scaffold operation should still succeed if there is no 'git' executable.
+   * The scaffold operation should still succeed if there is no 'git'
+   * executable.
    */
   public function testUnmanagedGitIgnoreWhenGitNotAvailable() {
-    // Note that the drupal-composer-drupal-project fixture does not
-    // have any configuration settings related to .gitignore management.
+    // Note that the drupal-composer-drupal-project fixture does not have any
+    // configuration settings related to .gitignore management.
     $sut = $this->createSutWithGit('drupal-composer-drupal-project');
     $this->assertFileNotExists($sut . '/docroot/sites/default/.gitignore');
     $this->assertFileNotExists($sut . '/docroot/index.php');
     $this->assertFileNotExists($sut . '/docroot/sites/.gitignore');
-    // Confirm that 'git' is available (n.b. if it were not, createSutWithGit() would fail).
+    // Confirm that 'git' is available (n.b. if it were not, createSutWithGit()
+    // would fail).
     exec('git --help', $output, $status);
     $this->assertEquals(0, $status);
-    // Modify our $PATH so that it begins with a path that contains an executable
-    // script named 'git' that always exits with 127, as if git were not found.
-    // Note that we run our tests using process isolation, so we do not need to
-    // restore the PATH when we are done.
+    // Modify our $PATH so that it begins with a path that contains an
+    // executable script named 'git' that always exits with 127, as if git were
+    // not found. Note that we run our tests using process isolation, so we do
+    // not need to restore the PATH when we are done.
     $unavailableGitPath = $this->fixtures->binFixtureDir('disable-git-bin');
     chmod($unavailableGitPath . '/git', 0755);
     putenv('PATH=' . $unavailableGitPath . ':' . getenv('PATH'));
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php
index ec6eae0c28..3e733c2ad2 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php
@@ -17,7 +17,7 @@
  */
 class ScaffoldTest extends TestCase {
   use AssertUtilsTrait;
-  const FIXTURE_DIR = 'SCAFFOLD_FIXTURE_DIR';
+
   /**
    * The root of this project.
    *
@@ -27,18 +27,14 @@ class ScaffoldTest extends TestCase {
    * @var string
    */
   protected $projectRoot;
+
   /**
    * Directory to perform the tests in.
    *
    * @var string
    */
   protected $fixturesDir;
-  /**
-   * The file path to the system under test.
-   *
-   * @var string
-   */
-  protected $sut;
+
   /**
    * The Symfony FileSystem component.
    *
@@ -46,6 +42,13 @@ class ScaffoldTest extends TestCase {
    */
   protected $fileSystem;
 
+  /**
+   * The Fixtures object.
+   *
+   * @var \Drupal\Tests\Component\Scaffold\Fixtures
+   */
+  protected $fixtures;
+
   /**
    * {@inheritdoc}
    */
@@ -53,7 +56,10 @@ protected function setUp() {
     $this->fileSystem = new Filesystem();
     $this->fixtures = new Fixtures();
     $this->projectRoot = $this->fixtures->projectRoot();
-    $this->fixturesDir = getenv(self::FIXTURE_DIR);
+    // The directory used for creating composer projects to test can be
+    // configured using the SCAFFOLD_FIXTURE_DIR environment variable. Otherwise
+    // a directory will be created in the system's temporary directory.
+    $this->fixturesDir = getenv('SCAFFOLD_FIXTURE_DIR');
     if (!$this->fixturesDir) {
       $this->fixturesDir = $this->fixtures->tmpDir($this->getName());
     }
@@ -68,15 +74,25 @@ protected function tearDown() {
   }
 
   /**
-   * Create the System-Under-Test.
+   * Creates the System-Under-Test.
+   *
+   * @param string $fixture_name
+   *   The name of the fixture to use from
+   *   core/tests/Drupal/Tests/Component/Scaffold/fixtures.
+   * @param array $replacements
+   *   Key : value mappings for placeholders to replace in composer.json
+   *   templates.
+   *
+   * @return string
+   *   The path to the created System-Under-Test.
    */
-  protected function createSut($topLevelProjectDir, $replacements = []) {
-    $this->sut = $this->fixturesDir . '/' . $topLevelProjectDir;
+  protected function createSut($fixture_name, array $replacements = []) {
+    $sut = $this->fixturesDir . '/' . $fixture_name;
     // Erase just our sut, to ensure it is clean. Recopy all of the fixtures.
-    $this->fileSystem->remove($this->sut);
+    $this->fileSystem->remove($sut);
     $replacements += ['PROJECT_ROOT' => $this->projectRoot];
     $this->fixtures->cloneFixtureProjects($this->fixturesDir, $replacements);
-    return $this->sut;
+    return $sut;
   }
 
   /**
@@ -95,15 +111,23 @@ public function scaffoldFixturesWithErrorConditionsTestValues() {
   /**
    * Tests that scaffold files throw when they have bad values.
    *
+   * @param string $fixture_name
+   *   The name of the fixture to use from
+   *   core/tests/Drupal/Tests/Component/Scaffold/fixtures.
+   * @param string $expected_exception_message
+   *   The expected exception message.
+   * @param bool $is_link
+   *   Whether or not symlinking should be used.
+   *
    * @dataProvider scaffoldFixturesWithErrorConditionsTestValues
    */
-  public function testScaffoldFixturesWithErrorConditions($topLevelProjectDir, $expectedExceptionMessage, $is_link) {
-    $sut = $this->createSut($topLevelProjectDir, ['SYMLINK' => $is_link ? 'true' : 'false']);
+  public function testScaffoldFixturesWithErrorConditions($fixture_name, $expected_exception_message, $is_link) {
+    $sut = $this->createSut($fixture_name, ['SYMLINK' => $is_link ? 'true' : 'false']);
     // Run composer install to get the dependencies we need to test.
-    $this->fixtures->runComposer("install --no-ansi --no-scripts", $this->sut);
+    $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     // Test scaffold. Expect an error.
     $this->expectException(\Exception::class);
-    $this->expectExceptionMessage($expectedExceptionMessage);
+    $this->expectExceptionMessage($expected_exception_message);
     $this->fixtures->runScaffold($sut);
   }
 
@@ -142,16 +166,25 @@ public function scaffoldTestValues() {
   /**
    * Tests that scaffold files are correctly moved.
    *
+   * @param string $fixture_name
+   *   The name of the fixture to use from
+   *   core/tests/Drupal/Tests/Component/Scaffold/fixtures.
+   * @param string $assert_function
+   *   The name of the assert function to call after running the scaffold
+   *   command.
+   * @param bool $is_link
+   *   Whether to use symlinking.
+   *
    * @dataProvider scaffoldTestValues
    */
-  public function testScaffold($topLevelProjectDir, $scaffoldAssertions, $is_link) {
-    $sut = $this->createSut($topLevelProjectDir, ['SYMLINK' => $is_link ? 'true' : 'false']);
+  public function testScaffold($fixture_name, $assert_function, $is_link) {
+    $sut = $this->createSut($fixture_name, ['SYMLINK' => $is_link ? 'true' : 'false']);
     // Run composer install to get the dependencies we need to test.
-    $this->fixtures->runComposer("install --no-ansi --no-scripts", $this->sut);
+    $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     // Test composer:scaffold.
-    $scaffoldOutput = $this->fixtures->runScaffold($sut);
+    $this->fixtures->runScaffold($sut);
     // @todo We could assert that $scaffoldOutput must contain some expected text
-    call_user_func([$this, $scaffoldAssertions], $sut, $is_link, $topLevelProjectDir);
+    call_user_func([$this, $assert_function], $sut, $is_link, $fixture_name);
   }
 
   /**
@@ -161,7 +194,7 @@ public function testEmptyProject() {
     $topLevelProjectDir = 'empty-fixture';
     $sut = $this->createSut($topLevelProjectDir, ['SYMLINK' => 'false']);
     // Run composer install to get the dependencies we need to test.
-    $this->fixtures->runComposer("install --no-ansi --no-scripts", $this->sut);
+    $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     // Test composer:scaffold.
     $scaffoldOutput = $this->fixtures->runScaffold($sut);
     $this->assertContains('Nothing scaffolded because no packages are allowed in the top-level composer.json file', $scaffoldOutput);
@@ -174,12 +207,12 @@ public function testProjectThatScaffoldsEmptyProject() {
     $topLevelProjectDir = 'project-allowing-empty-fixture';
     $sut = $this->createSut($topLevelProjectDir, ['SYMLINK' => 'false']);
     // Run composer install to get the dependencies we need to test.
-    $this->fixtures->runComposer("install --no-ansi --no-scripts", $this->sut);
+    $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     // Test composer:scaffold.
     $scaffoldOutput = $this->fixtures->runScaffold($sut);
     $this->assertContains('The allowed package fixtures/empty-fixture does not provide a file mapping for Composer Scaffold', $scaffoldOutput);
     $docroot = $sut;
-    $this->assertCommonDrupalAssetsWereScaffolded($docroot, FALSE, $topLevelProjectDir);
+    $this->assertCommonDrupalAssetsWereScaffolded($docroot, FALSE);
   }
 
   /**
@@ -189,7 +222,7 @@ public function testProjectWithEmptyScaffoldPath() {
     $topLevelProjectDir = 'project-with-empty-scaffold-path';
     $sut = $this->createSut($topLevelProjectDir, ['SYMLINK' => 'false']);
     // Run composer install to get the dependencies we need to test.
-    $this->fixtures->runComposer("install --no-ansi --no-scripts", $this->sut);
+    $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     // Test composer:scaffold.
     $this->expectException(\Exception::class);
     $this->expectExceptionMessage('No scaffold file path given for [web-root]/my-error in package fixtures/project-with-empty-scaffold-path');
@@ -203,7 +236,7 @@ public function testProjectWithIllegalDirScaffold() {
     $topLevelProjectDir = 'project-with-illegal-dir-scaffold';
     $sut = $this->createSut($topLevelProjectDir, ['SYMLINK' => 'false']);
     // Run composer install to get the dependencies we need to test.
-    $this->fixtures->runComposer("install --no-ansi --no-scripts", $this->sut);
+    $this->fixtures->runComposer("install --no-ansi --no-scripts", $sut);
     // Test composer:scaffold.
     $this->expectException(\Exception::class);
     $this->expectExceptionMessage('Scaffold file assets in package fixtures/project-with-illegal-dir-scaffold is a directory; only files may be scaffolded');
@@ -211,34 +244,53 @@ public function testProjectWithIllegalDirScaffold() {
   }
 
   /**
-   * Asserts that the drupal/assets scaffold files correct for drupal/project layout.
+   * Asserts that the drupal/assets scaffold files correct for drupal/project.
+   *
+   * @param string $sut
+   *   The path to the System-under-Test.
+   * @param bool $is_link
+   *   Whether or not symlinking is used.
+   * @param string $project_name
+   *   Not used but this assertion is called dynamically and this argument is
+   *   provided.
    */
   protected function assertDrupalProjectSutWasScaffolded($sut, $is_link, $project_name) {
     $docroot = $sut . '/docroot';
-    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link, $project_name);
+    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link);
     $this->assertDefaultSettingsFromScaffoldOverride($docroot, $is_link);
     $this->assertHtaccessExcluded($docroot);
   }
 
   /**
-   * Asserts that the drupal/assets scaffold files correct for drupal/drupal layout.
+   * Asserts that the drupal/assets scaffold files correct for drupal/drupal.
+   *
+   * @param string $sut
+   *   The path to the System-under-Test.
+   * @param bool $is_link
+   *   Whether or not symlinking is used.
+   * @param string $project_name
+   *   Not used but this assertion is called dynamically and this argument is
+   *   provided.
    */
   protected function assertDrupalDrupalSutWasScaffolded($sut, $is_link, $project_name) {
     $docroot = $sut;
-    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link, $project_name);
+    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link);
     $this->assertDefaultSettingsFromScaffoldOverride($docroot, $is_link);
     $this->assertHtaccessExcluded($docroot);
   }
 
   /**
-   * Ensure that the default settings file was overridden by the test.
+   * Asserts that the default settings file was overridden by the test.
    */
   protected function assertDefaultSettingsFromScaffoldOverride($docroot, $is_link) {
     $this->assertScaffoldedFile($docroot . '/sites/default/default.settings.php', $is_link, '#scaffolded from the scaffold-override-fixture#');
   }
 
   /**
-   * Ensure that the .htaccess file was excluded by the test.
+   * Asserts that the .htaccess file was excluded by the test.
+   *
+   * @param string $docroot
+   *   The path to the System-under-Test's docroot.
    */
   protected function assertHtaccessExcluded($docroot) {
     // Ensure that the .htaccess.txt file was not written, as our
@@ -247,39 +299,58 @@ protected function assertHtaccessExcluded($docroot) {
   }
 
   /**
-   * Assert that the appropriate file was replaced.
+   * Asserts that the appropriate file was replaced.
    *
    * Check the drupal/drupal-based project to confirm that the expected file was
    * replaced, and that files that were not supposed to be replaced remain
    * unchanged.
+   *
+   * @param string $sut
+   *   The path to the System-under-Test.
+   * @param bool $is_link
+   *   Whether or not symlinking is used.
+   * @param string $project_name
+   *   The project name expected in the robots.txt file.
    */
   protected function assertDrupalDrupalFileWasReplaced($sut, $is_link, $project_name) {
     $docroot = $sut;
     $this->assertScaffoldedFile($docroot . '/replace-me.txt', $is_link, '#from assets that replaces file#');
     $this->assertScaffoldedFile($docroot . '/keep-me.txt', $is_link, '#File in drupal-drupal-test-overwrite that is not replaced#');
     $this->assertScaffoldedFile($docroot . '/make-me.txt', $is_link, '#from assets that replaces file#');
-    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link, $project_name);
+    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link);
     $this->assertScaffoldedFile($docroot . '/robots.txt', $is_link, "#{$project_name}#");
   }
 
   /**
-   * Confirm that the robots.txt file was prepended / appended as stipulated in the test.
+   * Asserts that the robots.txt file was prepended / appended as expected.
+   *
+   * @param string $sut
+   *   The path to the System-under-Test.
+   * @param bool $is_link
+   *   Whether or not symlinking is used.
+   * @param string $project_name
+   *   Not used but this assertion is called dynamically and this argument is
+   *   provided.
    */
   protected function assertDrupalDrupalFileWasAppended($sut, $is_link, $project_name) {
     $docroot = $sut;
     $this->assertScaffoldedFile($docroot . '/robots.txt', FALSE, '#in drupal-drupal-test-append composer.json fixture.*This content is prepended to the top of the existing robots.txt fixture.*Test version of robots.txt from drupal/core.*This content is appended to the bottom of the existing robots.txt fixture.*in drupal-drupal-test-append composer.json fixture#ms');
-    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link, $project_name);
+    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link);
   }
 
   /**
-   * Assert that the scaffold files from drupal/assets are placed as we expect them to be.
+   * Asserts that the scaffold files from drupal/assets are placed as expected.
    *
    * This tests that all assets from drupal/assets were scaffolded, save
    * for .htaccess, robots.txt and default.settings.php, which are scaffolded
    * in different ways in different tests.
+   *
+   * @param string $docroot
+   *   The path to the System-under-Test's docroot.
+   * @param bool $is_link
+   *   Whether or not symlinking is used.
    */
-  protected function assertCommonDrupalAssetsWereScaffolded($docroot, $is_link, $project_name) {
-    $from_project = "#scaffolded from \"file-mappings\" in {$project_name} composer.json fixture#";
+  protected function assertCommonDrupalAssetsWereScaffolded($docroot, $is_link) {
     $from_core = '#from drupal/core#';
     // Ensure that the autoload.php file was written.
     $this->assertFileExists($docroot . '/autoload.php');
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Integration/AppendOpTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Integration/AppendOpTest.php
index 75b8be42d4..e381cd59ce 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Integration/AppendOpTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Integration/AppendOpTest.php
@@ -20,7 +20,7 @@ class AppendOpTest extends TestCase {
   public function testProcess() {
     $fixtures = new Fixtures();
     $destination = $fixtures->destinationPath('[web-root]/robots.txt');
-    $options = ScaffoldOptions::defaultOptions();
+    $options = ScaffoldOptions::create([]);
     // Assert that there is no target file before we run our test.
     $this->assertFileNotExists($destination->fullPath());
 
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Integration/OperationCollectionTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Integration/OperationCollectionTest.php
index e4b8b23f71..2a690a0e6f 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Integration/OperationCollectionTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Integration/OperationCollectionTest.php
@@ -20,7 +20,7 @@ class OperationCollectionTest extends TestCase {
   /**
    * @covers ::collateScaffoldFiles
    */
-  public function testCoalateScaffoldFiles() {
+  public function testCollateScaffoldFiles() {
     $fixtures = new Fixtures();
     $locationReplacements = $fixtures->getLocationReplacements();
     $file_mappings = [
@@ -41,9 +41,11 @@ public function testCoalateScaffoldFiles() {
     $sut = new OperationCollection($fixtures->io());
     // Test the system under test.
     list($scaffold_list, $resolved_file_mappings) = $this->callProtected($sut, 'collateScaffoldFiles', [$file_mappings, $locationReplacements]);
-    // Confirm that the keys of the output are the same as the keys of the input.
+    // Confirm that the keys of the output are the same as the keys of the
+    // input.
     $this->assertEquals(array_keys($file_mappings), array_keys($resolved_file_mappings));
-    // Also assert that we have the right ScaffoldFileInfo objects in the destination.
+    // Also assert that we have the right ScaffoldFileInfo objects in the
+    // destination.
     $this->assertResolvedToSameOp('fixtures/drupal-assets-fixture', '[web-root]/index.php', $file_mappings, $scaffold_list, $resolved_file_mappings);
     $this->assertResolvedToSameOp('fixtures/drupal-profile', '[web-root]/sites/default/default.services.yml', $file_mappings, $scaffold_list, $resolved_file_mappings);
     $this->assertResolvedToSameOp('fixtures/drupal-drupal', '[web-root]/robots.txt', $file_mappings, $scaffold_list, $resolved_file_mappings);
@@ -53,16 +55,27 @@ public function testCoalateScaffoldFiles() {
   }
 
   /**
-   * Check to see if a given file was not overridden.
+   * Checks to see if a given file was not overridden.
    *
    * The package name in the scaffold list for the provided destination should
    * match the package name from the specified project.
+   *
+   * @param string $project
+   *   The project to check.
+   * @param string $dest
+   *   The destination to check.
+   * @param array $file_mappings
+   *   The test file mappings keyed by project and destination.
+   * @param \Drupal\Component\Scaffold\ScaffoldFileInfo[] $scaffold_list
+   *   The list of scaffolded files keyed by destination.
+   * @param array $resolved_file_mappings
+   *   The list of resolved file mappings keyed by project and destination.
    */
-  protected function assertResolvedToSameOp($project, $dest, $file_mappings, $scaffold_list, $resolved_file_mappings) {
+  protected function assertResolvedToSameOp($project, $dest, array $file_mappings, array $scaffold_list, array $resolved_file_mappings) {
     $resolved_file_info = $resolved_file_mappings[$project][$dest];
     $this->assertEquals(get_class($resolved_file_info), ScaffoldFileInfo::class);
     $resolved_scaffold_op = $resolved_file_info->op();
-    // If this is an append op then it will be overridden by a preprocess op
+    // If this is an append op then it will be part of a conjunction op.
     $expected = get_class($file_mappings[$project][$dest]);
     if ($expected == AppendOp::class) {
       $this->assertEquals(ConjunctionOp::class, get_class($resolved_scaffold_op));
@@ -75,12 +88,21 @@ protected function assertResolvedToSameOp($project, $dest, $file_mappings, $scaf
   }
 
   /**
-   * Check if a given file was overridden.
+   * Checks if a given file was overridden.
    *
    * Assert that the file in the scaffold list at the specified destination
    * comes from a different package than the one in the file info.
+   *
+   * @param string $project
+   *   The project to check.
+   * @param string $dest
+   *   The destination to check.
+   * @param \Drupal\Component\Scaffold\ScaffoldFileInfo[] $scaffold_list
+   *   The list of scaffolded files keyed by destination.
+   * @param array $resolved_file_mappings
+   *   The list of resolved file mappings keyed by project and destination.
    */
-  protected function assertOverridden($project, $dest, $scaffold_list, $resolved_file_mappings) {
+  protected function assertOverridden($project, $dest, array $scaffold_list, array $resolved_file_mappings) {
     $resolved_file_info = $resolved_file_mappings[$project][$dest];
     $this->assertEquals(get_class($resolved_file_info), ScaffoldFileInfo::class);
     $this->assertNotEquals($project, $scaffold_list[$dest]->packageName());
@@ -92,14 +114,14 @@ protected function assertOverridden($project, $dest, $scaffold_list, $resolved_f
    * @param mixed $obj
    *   The object to inspect.
    * @param string $methodName
-   *   The name of the method to call
+   *   The name of the method to call.
    * @param array $args
-   *   The arguments to pass to the protected method
+   *   The arguments to pass to the protected method.
    *
    * @return mixed
-   *   The return value from the protected methos.
+   *   The return value from the protected method.
    */
-  protected function callProtected($obj, $methodName, $args = []) {
+  protected function callProtected($obj, $methodName, array $args = []) {
     $r = new \ReflectionMethod($obj, $methodName);
     $r->setAccessible(TRUE);
     return $r->invokeArgs($obj, $args);
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Integration/ReplaceOpTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Integration/ReplaceOpTest.php
index ec568fc8da..a45d25635e 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Integration/ReplaceOpTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Integration/ReplaceOpTest.php
@@ -21,7 +21,7 @@ public function testProcess() {
     $fixtures = new Fixtures();
     $destination = $fixtures->destinationPath('[web-root]/robots.txt');
     $source = $fixtures->sourcePath('drupal-assets-fixture', 'robots.txt');
-    $options = ScaffoldOptions::defaultOptions();
+    $options = ScaffoldOptions::create([]);
     $sut = new ReplaceOp($source, TRUE);
     // Assert that there is no target file before we run our test.
     $this->assertFileNotExists($destination->fullPath());
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Integration/SkipOpTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Integration/SkipOpTest.php
index fb600bd935..27e358e6bf 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Integration/SkipOpTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Integration/SkipOpTest.php
@@ -20,8 +20,7 @@ class SkipOpTest extends TestCase {
   public function testProcess() {
     $fixtures = new Fixtures();
     $destination = $fixtures->destinationPath('[web-root]/robots.txt');
-    $source = $fixtures->sourcePath('drupal-assets-fixture', 'robots.txt');
-    $options = ScaffoldOptions::defaultOptions();
+    $options = ScaffoldOptions::create([]);
     $sut = new SkipOp();
     // Assert that there is no target file before we run our test.
     $this->assertFileNotExists($destination->fullPath());
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/README.md b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/README.md
index e103ffba5c..0c8ab5bf80 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/README.md
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/README.md
@@ -1,8 +1,12 @@
 # Fixtures README
 
-These fixtures are automatically copied to a temporary directory during test runs. After the test run, the fixtures are automatically deleted.
+These fixtures are automatically copied to a temporary directory during test
+runs. After the test run, the fixtures are automatically deleted.
 
-Set the SCAFFOLD_FIXTURE_DIR environment variable to place the fixtures in a specific location rather than a temporary directory. If this is done, then the fixtures will not be deleted after the test run. This is useful for ad-hoc testing.
+Set the SCAFFOLD_FIXTURE_DIR environment variable to place the fixtures in a
+specific location rather than a temporary directory. If this is done, then the
+fixtures will not be deleted after the test run. This is useful for ad-hoc
+testing.
 
 Example:
 
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/scaffold-override-fixture/assets/override-settings.php b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/scaffold-override-fixture/assets/override-settings.php
index f37a684279..064ed7e3f5 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/scaffold-override-fixture/assets/override-settings.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/scaffold-override-fixture/assets/override-settings.php
@@ -2,5 +2,5 @@
 
 /**
  * @file
- * This is a settings.php fixture file scaffolded from the scaffold-override-fixture.
+ * A settings.php fixture file scaffolded from the scaffold-override-fixture.
  */