Fix timing in class type-checking

(make sure the timing goes away if the flag is off)

original commit: ee3e83856c6f6973c29ada0c809a5ecb50d871aa

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/check-class-unit.rkt

@@ -43,9 +43,14 @@
                e ...))
          #'(begin e ...))]))
 
-(define (do-timestamp [str ""])
-  (log-info (format "TR class time @ ~a: ~a"
-                    str (- (current-inexact-milliseconds) (start-time)))))
+(define-syntax (do-timestamp stx)
+  (syntax-case stx ()
+    [(_ str)
+     (if (syntax-local-value #'do-timing)
+         #'(log-info
+            (format "TR class time @ ~a: ~a"
+                    str (- (current-inexact-milliseconds) (start-time))))
+         #'(void))]))
 
 ;; Syntax classes for use in functions below
 (define-syntax-class name-pair