fix #29228, let* precedence should match generated expression

tests/language_strong/await_and_ifnull_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+
+Future<String> foo(bool x) async => x ? "foo" : null;
+
+Future<String> bar(bool x) async {
+  return ((await foo(x)) ?? "bar").toUpperCase();
+}
+
+main() async {
+  Expect.equals(await bar(true), "FOO");
+  Expect.equals(await bar(false), "BAR");
+}