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/perf/hwtest.js

@@ -57,7 +57,7 @@ function waitAndDraw(milliseconds) {
             cb();
     });
 
-    return callback => cb = callback;
+    return callback => (cb = callback);
 }
 
 function waitSignal(object, signal) {
@@ -69,7 +69,7 @@ function waitSignal(object, signal) {
             cb();
     });
 
-    return callback => cb = callback;
+    return callback => (cb = callback);
 }
 
 function extractBootTimestamp() {