Pass the entire Composer event to the downloadScaffold method.

src/Handler.php

@@ -88,7 +88,7 @@ public function onPostCmdEvent(\Composer\Script\Event $event) {
     // Only install the scaffolding if drupal/core was installed,
     // AND there are no scaffolding files present.
     if (isset($this->drupalCorePackage)) {
-      $this->downloadScaffold($event->isDevMode());
+      $this->downloadScaffold($event);
       // Generate the autoload.php file after generating the scaffold files.
       $this->generateAutoload();
     }
@@ -97,18 +97,18 @@ public function onPostCmdEvent(\Composer\Script\Event $event) {
   /**
    * Downloads drupal scaffold files for the current process.
    *
-   * @param bool $dev
-   *   TRUE if dev packages are installed. FALSE otherwise.
+   * @param \Composer\Script\Event $event
+   *   The Composer event.
    */
-  public function downloadScaffold($dev = FALSE) {
+  public function downloadScaffold($event) {
     $drupalCorePackage = $this->getDrupalCorePackage();
     $webroot = realpath($this->getWebRoot());
 
     // Collect options, excludes, dev and settings files.
     $options = $this->getOptions();
     $includes = $this->getIncludes();
     // Check dev files if necessary.
-    if ($dev) {
+    if ($event->isDevMode()) {
       $includes = array_merge($includes, $this->getIncludesDev());
     }
     $files = array_diff($includes, $this->getExcludes());

src/Plugin.php

@@ -74,7 +74,7 @@ public function postCmd(\Composer\Script\Event $event) {
    */
   public static function scaffold(\Composer\Script\Event $event) {
     $handler = new Handler($event->getComposer(), $event->getIO());
-    $handler->downloadScaffold($event->isDevMode());
+    $handler->downloadScaffold($event);
     // Generate the autoload.php file after generating the scaffold files.
     $handler->generateAutoload();
   }