clang-format: [JS] Support index signature types.

Patch by Martin Probst.

llvm-svn: 234754
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
index eb587f2..3c8c14d 100644
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -670,5 +670,9 @@
                "}");
 }
 
+TEST_F(FormatTestJS, IndexSignature) {
+  verifyFormat("var x: {[k: string]: v};");
+}
+
 } // end namespace tooling
 } // end namespace clang