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

@@ -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) {