cleanup: Use default parameters where appropriate

Since ES6 it is possible to set an explicit default value for optional
parameters (overriding the implicit value of 'undefined'). Use them
for a nice small cleanup.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/626

js/ui/panel.js

@@ -706,9 +706,7 @@ var PanelCorner = class {
 
 var AggregateLayout = GObject.registerClass(
 class AggregateLayout extends Clutter.BoxLayout {
-    _init(params) {
-        if (!params)
-            params = {};
+    _init(params = {}) {
         params['orientation'] = Clutter.Orientation.VERTICAL;
         super._init(params);