[esnext] prototype runtime implementation for async functions

src/js/harmony-async-await.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.
+
+(function(global, utils, extrasUtils) {
+
+"use strict";
+
+%CheckIsBootstrapping();
+
+// -------------------------------------------------------------------
+// Imports
+
+var AsyncFunctionNext;
+var AsyncFunctionThrow;
+var PromiseReject;
+var PromiseResolve;
+var PromiseThen;
+
+utils.Import(function(from) {
+  AsyncFunctionNext = from.AsyncFunctionNext;
+  AsyncFunctionThrow = from.AsyncFunctionThrow;
+  PromiseReject = from.PromiseCreateRejected;
+  PromiseResolve = from.PromiseCreateResolved;
+  PromiseThen = from.PromiseThen;
+});
+
+// -------------------------------------------------------------------
+
+function AsyncFunctionAwait(generator, value) {
+  return %_Call(
+      PromiseThen, PromiseResolve(value),
+      function(sentValue) {
+        return %_Call(AsyncFunctionNext, generator, sentValue);
+      },
+      function(sentError) {
+        return %_Call(AsyncFunctionThrow, generator, sentError);
+      });
+}
+
+%InstallToContext([ "async_function_await", AsyncFunctionAwait ]);
+
+})