Fixing tests

src/future/future/mod.rs

@@ -365,8 +365,11 @@ extension_trait! {
             ```
             # async_std::task::block_on(async {  
             #
+            use std::time::Duration;
+
             use async_std::prelude::*;
             use async_std::future;
+            use async_std::task;
             
             let fut = future::ready(0);
             let dur = Duration::from_millis(100);