cleanup: Disambiguate assignments in arrow functions

As arrow functions have an implicit return value, an assignment of
this.foo = bar could have been intended as a this.foo === bar
comparison. To catch those errors, we will disallow these kinds
of assignments unless they are marked explicitly by an extra pair
of parentheses.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/731

js/ui/status/system.js

@@ -33,7 +33,7 @@ var AltSwitcher = class {
 
         this.actor = new St.Bin();
         this.actor.connect('destroy', this._onDestroy.bind(this));
-        this.actor.connect('notify::mapped', () => this._flipped = false);
+        this.actor.connect('notify::mapped', () => (this._flipped = false));
     }
 
     _sync() {