cleanup: Port non-GObject classes to JS6 classes

ES6 finally adds standard class syntax to the language, so we can
replace our custom Lang.Class framework with the new syntax. Any
classes that inherit from GObject will need special treatment,
so limit the port to regular javascript classes for now.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/361

js/ui/altTab.js

@@ -411,10 +411,8 @@ var AppSwitcherPopup = new Lang.Class({
     }
 });
 
-var CyclerHighlight = new Lang.Class({
-    Name: 'CyclerHighlight',
-
-    _init() {
+class CyclerHighlight {
+    constructor() {
         this._window = null;
 
         this.actor = new St.Widget({ layout_manager: new Clutter.BinLayout() });
@@ -434,7 +432,7 @@ var CyclerHighlight = new Lang.Class({
         this.actor.connect('notify::allocation',
                            this._onAllocationChanged.bind(this));
         this.actor.connect('destroy', this._onDestroy.bind(this));
-    },
+    }
 
     set window(w) {
         if (this._window == w)
@@ -452,7 +450,7 @@ var CyclerHighlight = new Lang.Class({
             windowActor.hide();
 
         this._clone.source = windowActor;
-    },
+    }
 
     _onAllocationChanged() {
         if (!this._window) {
@@ -465,12 +463,12 @@ var CyclerHighlight = new Lang.Class({
             this._highlight.set_position(rect.x - x, rect.y - y);
             this._highlight.show();
         }
-    },
+    }
 
     _onDestroy() {
         this.window = null;
     }
-});
+};
 
 // We don't show an actual popup, so just provide what SwitcherPopup
 // expects instead of inheriting from SwitcherList