fix interaction of mixins and contracted classes

pkgs/racket-pkgs/racket-test/tests/racket/contract/class.rkt

@@ -2295,6 +2295,23 @@
       (with-method ([m (o m)])
         (m #f))))
   
+  (test/spec-passed
+   'mixin1
+   '((mixin () () (super-new)) 
+     (contract (class/c m)
+               (class object% (define/public (m x) x) (super-new))
+               'pos
+               'neg)))
+  
+  (test/spec-passed
+   'mixin2
+   '(send (new ((mixin () () (super-new)) 
+                (contract (class/c [m (->m integer? integer?)])
+                          (class object% (define/public (m x) x) (super-new))
+                          'pos
+                          'neg)))
+          m 1))
+  
   (let ([expected-given?
          (λ (exn) (and (regexp-match? #rx"callback: contract violation" (exn-message exn))
                        (regexp-match? #rx"expected: boolean[?]" (exn-message exn))

racket/collects/racket/private/class-internal.rkt

@@ -4340,7 +4340,7 @@ An example
 
 (define (check-mixin-super mixin-name super% from-ids)
   (let ([mixin-name (or mixin-name 'mixin)])
-    (unless (class? super%)
+    (unless (-class? super%)
       (obj-error mixin-name 
                  "argument is not a class"
                  "argument" super%))