Change which ExceptionEvents are triggered by 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 23 matching lines @@
   //     value => AsyncFunctionNext(value),
   //     error => AsyncFunctionThrow(error)
   // );
   var promise = PromiseCastResolved(value);
 
   var onFulfilled =
       (sentValue) => %_Call(AsyncFunctionNext, generator, sentValue);
   var onRejected =
       (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
 
-  var throwawayCapability = NewPromiseCapability(GlobalPromise);
+  // false debugEvent to avoid redundant ExceptionEvents
+  var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
   return PerformPromiseThen(promise, onFulfilled, onRejected,
                             throwawayCapability);
 }
 
 %InstallToContext([ "async_function_await", AsyncFunctionAwait ]);
 
 })