WIP [esnext] implement async iteration proposal

test/test262/local-tests/test/language/expressions/async-generators/expression-yield-as-statement.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+author: Caitlin Potter <caitp@igalia.com>
+esid: 14.4
+description: >
+  `yield` is a valid statement within async generator function bodies.
+flags: [async]
+---*/
+
+var g1 = async function*() { yield; };
+var g2 = async function*() { yield 1; };
+
+var iter1 = g1();
+iter1.next().then(function(result) {
+  assert.sameValue(
+    result.value, undefined, "Without right-hand-side: first result `value`");
+  assert.sameValue(
+    result.done, false, "Without right-hand-side: first result `done` flag");
+}).then(undefined, $DONE);
+iter1.next(function(result) {
+  assert.sameValue(
+    result.value, undefined, "Without right-hand-side: second result `value`");
+  assert.sameValue(
+    result.done, true, "Without right-hand-side: second result `done` flag");
+}).then(undefined, $DONE);
+
+var iter2 = g2();
+iter2.next().then(function(result) {
+  assert.sameValue(
+    result.value, 1, "With right-hand-side: first result `value`");
+  assert.sameValue(
+    result.done, false, "With right-hand-side: first result `done` flag");
+}).then(undefined, $DONE);
+iter2.next(function(result) {
+  assert.sameValue(
+    result.value, undefined, "With right-hand-side: second result `value`");
+  assert.sameValue(
+    result.done, true, "With right-hand-side: second result `done` flag");
+}).then($DONE, $DONE);