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

src/js/async-await.js

Index: src/js/async-await.js
diff --git a/src/js/async-await.js b/src/js/async-await.js
index a1cac0d5cd62a6bb6850197db285460cf06681ea..dd8f5c0f7e5b8a00c3f79180cc38933dc36ca23d 100644
--- a/src/js/async-await.js
+++ b/src/js/async-await.js
@@ -90,8 +90,11 @@ function AsyncFunctionAwait(generator, awaited, outerPromise) {
     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