Add compile fail test for async on Rust 2018

Author: Manishearth

src/test/compile-fail/edition-kw.rs

@@ -0,0 +1,18 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Zedition=2018 -Zunstable-options
+
+#![allow(unused)]
+
+pub fn main() {
+    let async = 1;
+    //~^ ERROR expected pattern, found reserved keyword `async`
+}