Update Tokio to Rust 2018 (#1082)

tokio-io/src/async_write.rs

@@ -1,9 +1,8 @@
+use crate::AsyncRead;
 use bytes::Buf;
-use futures::{Async, Poll};
+use futures::{try_ready, Async, Poll};
 use std::io as std_io;
 
-use AsyncRead;
-
 /// Writes bytes asynchronously.
 ///
 /// The trait inherits from `std::io::Write` and indicates that an I/O object is