diff -u b/core/lib/Drupal/Core/Extension/Extension.php b/core/lib/Drupal/Core/Extension/Extension.php
--- b/core/lib/Drupal/Core/Extension/Extension.php
+++ b/core/lib/Drupal/Core/Extension/Extension.php
@@ -161,6 +161,14 @@
       'pathname' => $this->pathname,
       'filename' => $this->filename,
     );
+
+    // @todo ThemeHandler::listInfo(), ThemeHandler::rebuildThemeData(), and
+    //   system_list() are adding custom properties to the Extension object.
+    $info = new \ReflectionObject($this);
+    foreach ($info->getProperties(\ReflectionProperty::IS_PUBLIC) as $property) {
+      $data[$property->getName()] = $property->getValue($this);
+    }
+
     return serialize($data);
   }
 
@@ -172,6 +180,14 @@
     $this->type = $data['type'];
     $this->pathname = $data['pathname'];
     $this->filename = $data['filename'];
+
+    // @todo ThemeHandler::listInfo(), ThemeHandler::rebuildThemeData(), and
+    //   system_list() are adding custom properties to the Extension object.
+    foreach ($data as $property => $value) {
+      if (!isset($this->$property)) {
+        $this->$property = $value;
+      }
+    }
   }
 
 }