Fix type of string->number

svn: r17313

original commit: b9fd34b925d37125553464bde37e0591dfaa16f8

collects/tests/typed-scheme/unit-tests/typecheck-tests.ss

@@ -759,6 +759,9 @@
                 (-polydots (a) ((list -Number) (a a) . ->... . -Boolean))]
         [tc-e (ann (error 'foo) (values Number Number)) #:ret (ret (list -Number -Number))]
         
+        [tc-e (string->number "123")
+              (t:Un (-val #f) -Number)]
+        
         [tc-e #{(make-hash) :: (HashTable Number Number)}
               (make-Hashtable -Number -Number)]
         #;[tc-err (let: ([fact : (Number -> Number) (lambda: ([n : Number]) (if (zero? n) 1 (* n (fact (- n 1)))))])

collects/typed-scheme/private/base-env.ss

@@ -290,7 +290,7 @@
   [-> -Bytes -Bytes])]
 
 [number->string (->opt N [N] -String)]
-[string->number (->opt -String [N] -String)]
+[string->number (->opt -String [N] (Un (-val #f) N))]
 
 [current-milliseconds (-> -Integer)]