WIP [esnext] implement async iteration proposal

test/test262/local-tests/test/language/statements/for-await-of/Symbol.iterator-async-generator.js

Index: test/test262/local-tests/test/language/statements/for-await-of/Symbol.iterator-async-generator.js
diff --git a/test/test262/local-tests/test/language/statements/for-await-of/Symbol.iterator-async-generator.js b/test/test262/local-tests/test/language/statements/for-await-of/Symbol.iterator-async-generator.js
new file mode 100644
index 0000000000000000000000000000000000000000..7b78680e62ab13b2959d15a062729caa8dbd30e0
--- /dev/null
+++ b/test/test262/local-tests/test/language/statements/for-await-of/Symbol.iterator-async-generator.js
@@ -0,0 +1,66 @@
+// Copyright (C) 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>
+description: >
+  If [Symbol.asyncIterator] is null on target of the IterationStatement,
+  invoke it [Symbol.iterator] instead.
+es6id: pending
+flags: [async]
+---*/
+
+var log = [];
+var iterable = Object.setPrototypeOf({
+  [Symbol.iterator]() {
+    log.push("iterable[@@iterator]()");
+    return this;
+  },
+  [Symbol.asyncIterator]: null,
+
+  next() {
+    log.push("iterable.next()");
+    if (this._i < this._items.length) {
+      var value = this._items[this._i++];
+      return { value: Promise.resolve(value), done: false };
+    }
+    return { value: undefined, done: true };
+  },
+
+  _i: 0,
+  _items: ["a", "b", "c"]
+}, null);
+
+async function* g(iterable) {
+  log.push("begin iteration");
+  var i = 0;
+  for await (var value of iterable) {
+    log.push("awaited value " + value);
+    ++i;
+  }
+  yield i;
+}
+
+var iter = g(iterable);
+iter.next().then(function(result) {
+  assert.sameValue(result.value, 3, "Visits all elements");
+  assert.sameValue(result.done, false);
+  log.push("done");
+}, $DONE).then(undefined, $DONE);
+
+iter.next().then(function(result) {
+  assert.sameValue(result.value, undefined);
+  assert.sameValue(result.done, true);
+  assert.sameValue(log.join("\n"), [
+    "begin iteration",
+    "iterable[@@iterator]()",
+    "iterable.next()",
+    "awaited value a",
+    "iterable.next()",
+    "awaited value b",
+    "iterable.next()",
+    "awaited value c",
+    "iterable.next()",
+    "done"
+  ].join("\n"));
+}, $DONE).then($DONE, $DONE);