Fix async/await memory leak

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 0d470bbe8c84300dfcd9213ba1becb016294bd40..a7934abec0d173fea2dab8f87ecdaff7d45a5a11 100644
--- a/src/js/harmony-async-await.js
+++ b/src/js/harmony-async-await.js
@@ -67,10 +67,22 @@ function AsyncFunctionAwait(generator, awaited, outerPromise) {
   // );
   var promise = PromiseCastResolved(awaited);
 
-  var onFulfilled =
-      (sentValue) => %_Call(AsyncFunctionNext, generator, sentValue);
-  var onRejected =
-      (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
+  var onFulfilled = (sentValue) => {

[adamk, 2016/09/15 22:43:20]

Nit: no need for parens while you're changing this

[Dan Ehrenberg, 2016/09/16 00:36:01]

Done

+    %_Call(AsyncFunctionNext, generator, sentValue);
+    // The resulting Promise is a throwaway, so it doesn't matter what it
+    // resolves to. What is important is that we don't end up keeping the
+    // whole chain of intermediate Promises alive by returning the value
+    // of AsyncFunctionNext, as that would create a memory leak.
+    return;
+  };
+  var onRejected = (sentError) => {

[adamk, 2016/09/15 22:43:20]

ditto

[Dan Ehrenberg, 2016/09/16 00:36:00]

Done

+    %_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);