[js-perf-test] Add a basic async-await microbenchmark

test/js-perf-test/AsyncAwait/native.js

+// Copyright 2016 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.
+
+
+new BenchmarkSuite('Native', [1000], [
+  new Benchmark('Basic', false, false, 0, Basic, Setup),
+]);
+
+var a,b,c,d,e,f,g,h,i,j,x;
+
+function Setup() {
+  x = Promise.resolve();
+
+  j = async function j() { return x; };
+  i = async function i() {
+    for (let i = 0; i < 9; i++) await j();

[gsathya, 2016/12/16 15:23:05]

This isn't semantically equivalent to the promise code. await j() waits until
the promise j() is resolved before the next iteration starts which isn't true
for Promise.all

[jgruber, 2016/12/19 08:31:01]

Done, removed Promise.all.

+    return await j();
+  };
+  h = async function h() { return await i(); };
+  g = async function g() { return await h(); };
+  f = async function f() { return await g(); };
+  e = async function e() { return await f(); };
+  d = async function d() { return await e(); };
+  c = async function c() { return await d(); };
+  b = async function b() { return await c(); };
+  a = async function a() { return await b(); };

[gsathya, 2016/12/16 15:23:05]

No need for any await statements here. "return await i()" is same as "return
i()" in async functions

[jgruber, 2016/12/19 08:31:01]

Done.

+

[gsathya, 2016/12/16 15:23:05]

i would rewrite this as 

async function a {
  await i();
  await h();
  await h();
  ....
}

[jgruber, 2016/12/19 08:31:01]

The intention was to have both function nesting (a->b->...->j) and functions
containing several async calls (i->j,j,...,j). Does that make sense?

+  %RunMicrotasks();
+}
+
+function Basic() {
+  a();
+  %RunMicrotasks();
+}