Make asyncjs Future[void] play nicely with last line discardable calls

tests/js/tasync.nim

@@ -3,6 +3,7 @@ discard """
   output: '''
 0
 x
+e
 '''
 """
 
@@ -12,15 +13,19 @@ import asyncjs
 # for js
 proc y(e: int): Future[string]
 
-proc x(e: int) {.async.} =
+proc e: int {.discardable.} =
+  echo "e"
+  return 2
+
+proc x(e: int): Future[void] {.async.} =
   var s = await y(e)
   echo s
+  e()
 
 proc y(e: int): Future[string] {.async.} =
   echo 0
   return "x"
 
 
-
 discard x(2)