[gnome-shell] messageTray: Use spread syntax to get map keys

[Florian Müllner <fmuellner src gnome org>]

commit a593e4587b30c9e8d9b01e4604bc6de437d713f5
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Jul 18 19:20:42 2017 +0200

    messageTray: Use spread syntax to get map keys
    
    It wasn't supported when the code was originally written, now the
    fill-in code isn't accepted anymore ...
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785084

 js/ui/messageTray.js |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/js/ui/messageTray.js b/js/ui/messageTray.js
index 63e6ca1..80c0dee 100644
--- a/js/ui/messageTray.js
+++ b/js/ui/messageTray.js
@@ -1042,7 +1042,7 @@ const MessageTray = new Lang.Class({
     },
 
     getSources: function() {
-        return [k for (k of this._sources.keys())];
+        return [...this._sources.keys()];
     },
 
     _onSourceEnableChanged: function(policy, source) {