GitHub for Mac is silly, this should've been in the last commit

src/Core/Notifications/Types/AlertableNotification.php

@@ -0,0 +1,32 @@
+<?php namespace Flarum\Core\Notifications\Types;
+
+interface AlertableNotification
+{
+    /**
+     * Get the data to be stored in the alert.
+     *
+     * @return array
+     */
+    public function getAlertData();
+
+    /**
+     * Get the user that sent the notification.
+     *
+     * @return \Flarum\Core\Models\User|null
+     */
+    public function getSender();
+
+    /**
+     * Get the model that the notification is about.
+     *
+     * @return \Flarum\Core\Models\Model
+     */
+    public function getSubject();
+
+    /**
+     * Get the class name of this notification type's subject model.
+     *
+     * @return string
+     */
+    public static function getSubjectModel();
+}