Add missing import

Author: garyb

src/Node/FS/Async.purs

@@ -25,6 +25,7 @@ module Node.FS.Async
   ) where
 
 import Control.Monad.Eff
+import Control.Monad.Eff.Unsafe (unsafeInterleaveEff)
 import Control.Monad.Eff.Exception
 import Data.Date
 import Data.Either
@@ -334,4 +335,4 @@ exists :: forall eff. FilePath
                    -> (Boolean -> Eff eff Unit)
                    -> Eff (fs :: FS | eff) Unit
 exists file cb = mkEff $ \_ -> runFn2
-  fs.exists file $ \b -> runPure (unsafeInterleaveEff (cb b))
+  fs.exists file $ \b -> runPure (unsafeInterleaveEff (cb b))