WIP [esnext] implement async iteration proposal

test/test262/local-tests/test/language/expressions/async-generators/expression-await-thenable-as-yield-operand.js

Index: test/test262/local-tests/test/language/expressions/async-generators/expression-await-thenable-as-yield-operand.js
diff --git a/test/test262/local-tests/test/language/expressions/async-generators/expression-await-thenable-as-yield-operand.js b/test/test262/local-tests/test/language/expressions/async-generators/expression-await-thenable-as-yield-operand.js
new file mode 100644
index 0000000000000000000000000000000000000000..5de8b4cc4c3bf0948d725cd65c17efc16b35c64d
--- /dev/null
+++ b/test/test262/local-tests/test/language/expressions/async-generators/expression-await-thenable-as-yield-operand.js
@@ -0,0 +1,31 @@
+// 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 thenable = {
+  then: function(resolve, reject) {
+    resolve("a");
+  }
+};
+
+var iter = (async function*() {
+  yield await thenable;
+})();
+
+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);