Preserve async markers when parsing js_ast

tests/language_strong/async_cascade_test.dart

+// Copyright (c) 2014, 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';
+
+// Note, this is a regression test for:
+// https://github.com/dart-lang/sdk/issues/29252
+class Foo {
+  Future _x;
+  int z;
+  List list = [];
+
+  Future foo() async {
+    _x ??= new Future(() async {
+      z = await new Future.value(42);
+      list = list.toList()..add(z);
+    });
+    await _x;
+    return list[0];
+  }
+}
+
+main() async {
+  var f = new Foo();
+  var result = await f.foo();
+  Expect.equals(42, result);
+}