[async await] fix async await desugaring

test/mjsunit/regress/regress-5896.js

+// Copyright 2017 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.
+
+// Flags: --allow-natives-syntax
+
+async function foo() {
+  try {
+    return 1;
+  } finally {
+    return Promise.resolve().then(() => { return 2; });
+  }
+}
+
+foo()
+  .then(value => { found = value; assertEquals(2, value) })
+  .catch((e) => { %AbortJS(''+e); });