clang-format: [JS] whitespace after async in arrow functions.

Summary:
Async arrow functions should be marked with a whitespace after the async keyword, before the parameter list:
    x = async () => foo();

Before:
    x = async() => foo();

This makes it easier to tell apart an async arrow function from a call to a function called async.

Reviewers: bkramer

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D30399

llvm-svn: 296330
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
index fe87bf2..112b124 100644
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -463,6 +463,8 @@
   verifyFormat("export async function f() {\n"
                "  return fetch(x);\n"
                "}");
+  verifyFormat("let x = async () => f();");
+  verifyFormat("let x = async();");
   verifyFormat("class X {\n"
                "  async asyncMethod() {\n"
                "    return fetch(1);\n"