class/c: Swap after, not before, adding #:important blame context

Swapping the blame before adding #:important context associates the
important party with the negative party for the purposes of picking
“contract violation” versus “broke its own contract” messages in error
reporting. Therefore, only swap after adding the context.

fixes #2531

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

@@ -2748,4 +2748,31 @@
                         'pos 'neg)])
       (new c% [a (lambda () 1)]
               [b (lambda () #f)])))
+
+  (test/neg-blame
+   'override-important
+   #:header-of-message "contract violation"
+   '(let* ([a% (contract (class/c (override [b (->m string?)]))
+                         (class object%
+                           (super-new)
+                           (abstract b)
+                           (define/public (a) (b)))
+                         'pos 'neg)]
+           [b% (class a%
+                 (super-new)
+                 (define/override (b) #f))])
+      (send (new b%) a)))
+
+  (test/neg-blame
+   'inner-important
+   #:header-of-message "contract violation"
+   '(let* ([a% (contract (class/c (inner [a (->m string?)]))
+                         (class object%
+                           (super-new)
+                           (define/pubment (a) (inner "" a)))
+                         'pos 'neg)]
+           [b% (class a%
+                 (super-new)
+                 (define/augment (a) #f))])
+      (send (new b%) a)))
   )

racket/collects/racket/private/class-c-old.rkt

@@ -459,7 +459,7 @@
       (for/list ([name (in-list (internal-class/c-inners internal-ctc))]
                  [c (in-list (internal-class/c-inner-contracts internal-ctc))])
         (and c
-             ((contract-late-neg-projection c) (blame-add-method-context bswap name)))))
+             ((contract-late-neg-projection c) (blame-swap (blame-add-method-context blame name))))))
     
     (define internal-field-projections
       (for/list ([f (in-list (internal-class/c-inherit-fields internal-ctc))]
@@ -515,7 +515,7 @@
       (for/list ([m (in-list (internal-class/c-overrides internal-ctc))]
                  [c (in-list (internal-class/c-override-contracts internal-ctc))])
         (and c
-             ((contract-late-neg-projection c) (blame-add-method-context bswap m)))))
+             ((contract-late-neg-projection c) (blame-swap (blame-add-method-context blame m))))))
     
     (define augment/augride-projections
       (for/list ([m (in-list (append (internal-class/c-augments internal-ctc)