Import async-trait crate.

Bug: 164106399
Bug: 158290206
Test: mm
Change-Id: I0c4a491b785134e4ac1d5572e5ff94b8699dc0bc
diff --git a/tests/ui/bare-trait-object.stderr b/tests/ui/bare-trait-object.stderr
new file mode 100644
index 0000000..98cf679
--- /dev/null
+++ b/tests/ui/bare-trait-object.stderr
@@ -0,0 +1,11 @@
+error: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/bare-trait-object.rs:11:16
+   |
+11 | impl Trait for Send + Sync {
+   |                ^^^^^^^^^^^ help: use `dyn`: `dyn Send + Sync`
+   |
+note: the lint level is defined here
+  --> $DIR/bare-trait-object.rs:1:9
+   |
+1  | #![deny(bare_trait_objects)]
+   |         ^^^^^^^^^^^^^^^^^^