Preserve async markers when parsing js_ast

tests/language_strong/async_cascade_test.dart

+import 'dart:async';
+
+import 'package:expect/expect.dart';
+
+class Foo {
+  Future _x;
+  int z;
+  List list = [];
+
+  Future foo() async {
+    _x ??= new Future(() async {

[Jennifer Messerly, 2017/04/04 23:34:24]

is it worth adding a comment about what's going on here?

// this is the outer metalet which incorrectly copies function* to function

[vsm, 2017/04/05 19:14:31]

Thanks - added comment pointing to #issue number for explanation.

+      z = await new Future.value(42);
+      list = list.toList()..add(z);

[Jennifer Messerly, 2017/04/04 23:34:24]

// this triggers a nested MetaLet which breaks the outer one

[vsm, 2017/04/05 19:14:31]

Ditto

+    });
+    await _x;
+    return list[0];
+  }
+}
+
+main() async {
+  var f = new Foo();
+  var result = await f.foo();
+  Expect.equals(42, result);
+}