Desugar async/await to create the resulting Promise upfront

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 GlobalPromise;
 var NewPromiseCapability;
 var PerformPromiseThen;
 var PromiseCastResolved;
+var RejectPromise;
 
 utils.Import(function(from) {
   AsyncFunctionNext = from.AsyncFunctionNext;
   AsyncFunctionThrow = from.AsyncFunctionThrow;
   GlobalPromise = from.GlobalPromise;
   NewPromiseCapability = from.NewPromiseCapability;
   PromiseCastResolved = from.PromiseCastResolved;
   PerformPromiseThen = from.PerformPromiseThen;
+  RejectPromise = from.RejectPromise;
 });
 
 // -------------------------------------------------------------------
 
 function AsyncFunctionAwait(generator, value) {
   // Promise.resolve(value).then(
   //     value => AsyncFunctionNext(value),
   //     error => AsyncFunctionThrow(error)
   // );
   var promise = PromiseCastResolved(value);
 
   var onFulfilled =
       (sentValue) => %_Call(AsyncFunctionNext, generator, sentValue);
   var onRejected =
       (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
 
   // false debugEvent to avoid redundant ExceptionEvents
   var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
   return PerformPromiseThen(promise, onFulfilled, onRejected,
                             throwawayCapability);
 }
 
-%InstallToContext([ "async_function_await", AsyncFunctionAwait ]);
+// How the parser rejects promises from async/await desugaring
+function RejectPromiseNoDebugEvent(promise, reason) {
+  return RejectPromise(promise, reason, false);
+}
+
+%InstallToContext([
+  "async_function_await", AsyncFunctionAwait,
+  "reject_promise_no_debug_event", RejectPromiseNoDebugEvent,
+]);
 
 })