[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 Normal;
+var Throw;
+var PromiseReject;
+var PromiseResolve;
+var PromiseThen;
+
+utils.Import(function(from) {
+  Normal = from.AsyncFunctionNext;
+  Throw = from.AsyncFunctionThrow;
+  PromiseReject = from.PromiseCreateRejected;
+  PromiseResolve = from.PromiseCreateResolved;
+  PromiseThen = from.PromiseThen;
+});
+
+
+function Resume(generator, sentValue, resumeFunc) {
+  try {
+    var result = resumeFunc(generator, sentValue);
+    if (result.done) return PromiseResolve(result.value);
+  } catch (error) {
+    return PromiseReject(error);
+  }
+
+  return %_Call(PromiseThen, PromiseResolve(result.value),
+      function(value) { return Resume(generator, value, Normal); },
+      function(error) { return Resume(generator, error, Throw); });
+}
+
+function Start(generator) {
+  return Resume(generator, UNDEFINED, Normal);
+}
+
+%InstallToContext([ "async_function_start", Start ]);
+
+})