[async-await] Don't create resolving callbacks for throwaway promises

src/js/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();
 
@@ skipping 72 matching lines @@
   };
   var onRejected = sentError => {
     %_Call(AsyncFunctionThrow, generator, sentError);
     // Similarly, returning the huge Promise here would cause a long
     // resolution chain to find what the exception to throw is, and
     // create a similar memory leak, and it does not matter what
     // sort of rejection this intermediate Promise becomes.
     return;
   }
 
-  // Just forwarding the exception, so no debugEvent for throwawayCapability
-  var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
+  // Just forwarding the exception, so no debugEvent for throwawayCapability.
+  // The resultCapability.promise is only ever fulfilled internally,
+  // so we don't need the closures to protect against accidentally
+  // calling them multiple times.
+  var throwawayCapability = NewPromiseCapability(GlobalPromise, false, true);
 
   // The Promise will be thrown away and not handled, but it shouldn't trigger
   // unhandled reject events as its work is done
   SET_PRIVATE(throwawayCapability.promise, promiseHasHandlerSymbol, true);
 
   if (DEBUG_IS_ACTIVE) {
     if (IsPromise(awaited)) {
       // Mark the reject handler callback to be a forwarding edge, rather
       // than a meaningful catch handler
       SET_PRIVATE(onRejected, promiseForwardingHandlerSymbol, true);
@@ skipping 62 matching lines @@
 
 %InstallToContext([
   "async_function_await_caught", AsyncFunctionAwaitCaught,
   "async_function_await_uncaught", AsyncFunctionAwaitUncaught,
   "reject_promise_no_debug_event", RejectPromiseNoDebugEvent,
   "async_function_promise_create", AsyncFunctionPromiseCreate,
   "async_function_promise_release", AsyncFunctionPromiseRelease,
 ]);
 
 })