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/modalDialog.js

@@ -253,7 +253,7 @@ var ModalDialog = GObject.registerClass({
             opacity: 0,
             duration: FADE_OUT_DIALOG_TIME,
             mode: Clutter.AnimationMode.EASE_OUT_QUAD,
-            onComplete: () => this.state = State.FADED_OUT
+            onComplete: () => (this.state = State.FADED_OUT)
         });
     }
 });