WIP [esnext] implement async iteration proposal

test/test262/local-tests/test/language/expressions/async-generators/expression-await-as-yield-operand.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: >
+  AwaitExpressions are valid operands to yield expressions.
+flags: [async]
+---*/
+
+var iter = (async function*() {
+  yield await "a";
+})();
+
+iter.next().then(function(result) {
+  assert.sameValue(result.value, "a", 'First result `value`');
+  assert.sameValue(result.done, false, 'First result `done` flag');
+}, $DONE);
+
+iter.next().then(function(result) {
+  assert.sameValue(result.value, undefined, 'Second result `value`');
+  assert.sameValue(result.done, true, 'Second result `done` flag');
+  $DONE();
+}, $DONE);