[modules] Add a simple micro-benchmark for import/export accesses.

test/js-perf-test/Modules/value.js

Index: test/js-perf-test/Modules/value.js
diff --git a/test/js-perf-test/Modules/value.js b/test/js-perf-test/Modules/value.js
new file mode 100644
index 0000000000000000000000000000000000000000..8c6a88e7d1ab1588d68a064277d3eb50affdf54e
--- /dev/null
+++ b/test/js-perf-test/Modules/value.js
@@ -0,0 +1,6 @@
+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export let value = 0;
+export function set(x) { value = x };