CyclerPopup: Check for type using constructor

Classes that are sub-classes of GObject don't use a constructor per se, so we
can't check for new.target (as this is undefined) in _init.

Then compare the current constructor name instead.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/503

js/ui/altTab.js

@@ -477,8 +477,8 @@ var CyclerList = GObject.registerClass({
 var CyclerPopup = GObject.registerClass(
 class CyclerPopup extends SwitcherPopup.SwitcherPopup {
     _init() {
-        if (new.target === CyclerPopup)
-            throw new TypeError('Cannot instantiate abstract class ' + new.target.name);
+        if (this.constructor.name === CyclerPopup.prototype.constructor.name)
+            throw new TypeError(`Cannot instantiate abstract class ${this.constructor.name}`);
 
         super._init();