fix some contract errors

original commit: 492cab22cbebcdfeed9a4bc467d147b4722308e9

collects/typed-scheme/types/union.ss

@@ -19,6 +19,8 @@
 (define (flat t)
   (match t
     [(Union: es) es]
+    [(Values: (list (Result: (Union: es) _ _))) es]
+    [(Values: (list (Result: t _ _))) (list t)]
     [_ (list t)]))    
 
 (define (remove-subtypes ts)

collects/typed-scheme/utils/utils.ss

@@ -160,7 +160,7 @@ at least theoretically.
 
 
 ;; turn contracts on and off - off by default for performance.
-(define-for-syntax enable-contracts? #t)
+(define-for-syntax enable-contracts? #f)
 (provide (for-syntax enable-contracts?) p/c w/c cnt d-s/c d/c)
 
 ;; these are versions of the contract forms conditionalized by `enable-contracts?'