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

src/js/harmony-async-await.js

Index: src/js/harmony-async-await.js
diff --git a/src/js/harmony-async-await.js b/src/js/harmony-async-await.js
index 4109b77fb54174366cdae648b0dae7fc0c5b02af..396ebdc750a8c865f8007592a9a7ce518ba88d9a 100644
--- a/src/js/harmony-async-await.js
+++ b/src/js/harmony-async-await.js
@@ -36,6 +36,8 @@ utils.Import(function(from) {
 var promiseAwaitHandlerSymbol = utils.ImportNow("promise_await_handler_symbol");
 var promiseHandledHintSymbol =
     utils.ImportNow("promise_handled_hint_symbol");
+var promiseHasHandlerSymbol =
+    utils.ImportNow("promise_has_handler_symbol");
 
 // -------------------------------------------------------------------
 
@@ -77,6 +79,11 @@ function AsyncFunctionAwait(generator, awaited, mark) {
 
   // 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);
 }