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 4109b77fb54174366cdae648b0dae7fc0c5b02af..761c554455f55bee5f4e705379ae3cdcb02cc962 100644
--- a/src/js/harmony-async-await.js
+++ b/src/js/harmony-async-await.js
@@ -64,10 +64,22 @@ function AsyncFunctionAwait(generator, awaited, mark) {
   // );
   var promise = PromiseCastResolved(awaited);
 
-  var onFulfilled =
-      (sentValue) => %_Call(AsyncFunctionNext, generator, sentValue);
-  var onRejected =
-      (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
+  var onFulfilled = sentValue => {
+    %_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 => {
+    %_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;
+  }
 
   if (mark && DEBUG_IS_ACTIVE && IsPromise(awaited)) {
     // Mark the reject handler callback such that it does not influence