Migrate language/async_backwards... ... language/async_star_take...

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);
-}