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/thunderbolt.js

@@ -260,7 +260,7 @@ var Indicator = class extends PanelMenu.SystemIndicator {
         if (!this._source) {
             this._source = new MessageTray.Source(_("Thunderbolt"),
                                                   'thunderbolt-symbolic');
-            this._source.connect('destroy', () => this._source = null);
+            this._source.connect('destroy', () => (this._source = null));
 
             Main.messageTray.add(this._source);
         }