Hid the SettableFuture of future {}.

This should prevent successful casts to type SettableFuture, meaning
client code can't access and complete the internal Future without
resorting to reflection..

integration/kotlinx-coroutines-guava/src/ListenableFuture.kt

@@ -46,14 +46,14 @@ public fun <T> CoroutineScope.future(
 ): ListenableFuture<T> {
  require(!start.isLazy) { "$start start is not supported" }
  val newContext = newCoroutineContext(context)
- // TODO: It'd be nice not to leak this SettableFuture reference, which is easily blind-cast.
  val future = SettableFuture.create<T>()
  val coroutine = ListenableFutureCoroutine(newContext, future)
  future.addListener(
  coroutine,
  MoreExecutors.directExecutor())
  coroutine.start(start, coroutine, block)
- return future
+ // Return hides the SettableFuture. This should prevent casting.
+ return object: ListenableFuture<T> by future {}
 }
 
 /**