async/await: Don't trigger uncaught rejection handlers on throwaway Promises

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();
 
@@ skipping 18 matching lines @@
   NewPromiseCapability = from.NewPromiseCapability;
   PromiseCreate = from.PromiseCreate;
   PerformPromiseThen = from.PerformPromiseThen;
   RejectPromise = from.RejectPromise;
   ResolvePromise = from.ResolvePromise;
 });
 
 var promiseAwaitHandlerSymbol = utils.ImportNow("promise_await_handler_symbol");
 var promiseHandledHintSymbol =
     utils.ImportNow("promise_handled_hint_symbol");
+var promiseHasHandlerSymbol =
+    utils.ImportNow("promise_has_handler_symbol");
 
 // -------------------------------------------------------------------
 
 function PromiseCastResolved(value) {
   if (IsPromise(value)) {
     return value;
   } else {
     var promise = PromiseCreate();
     ResolvePromise(promise, value);
     return promise;
@@ skipping 21 matching lines @@
       (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
 
   if (mark && DEBUG_IS_ACTIVE && IsPromise(awaited)) {
     // Mark the reject handler callback such that it does not influence
     // catch prediction.
     SET_PRIVATE(onRejected, promiseAwaitHandlerSymbol, true);
   }
 
   // Just forwarding the exception, so no debugEvent for throwawayCapability
   var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
+
+  // 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);
+
   return PerformPromiseThen(promise, onFulfilled, onRejected,
                             throwawayCapability);
 }
 
 // Called by the parser from the desugaring of 'await' when catch
 // prediction indicates no locally surrounding catch block
 function AsyncFunctionAwaitUncaught(generator, awaited) {
   // TODO(littledan): Install a dependency edge from awaited to outerPromise
   return AsyncFunctionAwait(generator, awaited, true);
 }
@@ skipping 12 matching lines @@
   return RejectPromise(promise, reason, false);
 }
 
 %InstallToContext([
   "async_function_await_caught", AsyncFunctionAwaitCaught,
   "async_function_await_uncaught", AsyncFunctionAwaitUncaught,
   "reject_promise_no_debug_event", RejectPromiseNoDebugEvent,
 ]);
 
 })