clang-format: [JS] fix async parsing.

Summary:
Only treat the sequence `async function` as the start of a function expression,
as opposed to every occurrence of the token `async` (whoops).

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D20737

llvm-svn: 271184
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
index 4572987..33fd065 100644
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -354,6 +354,10 @@
   verifyFormat("class X {\n"
                "  async asyncMethod() { return fetch(1); }\n"
                "}");
+  verifyFormat("function initialize() {\n"
+               "  // Comment.\n"
+               "  return async.then();\n"
+               "}\n");
 }
 
 TEST_F(FormatTestJS, ArrayLiterals) {