Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/run/ignores_explicit_dart2js_transformer_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("doesn't choke on an explicit dart2js transformer", () {
-    d.dir(appPath, [d.pubspec({
-        "name": "myapp",
-        "transformers": [r"$dart2js"]
-      }),
-          d.dir("bin", [d.file("script.dart", "main() => print('Hello!');")])]).create();
-
-    var pub = pubRun(args: ["script"]);
-    pub.stdout.expect("Hello!");
-    pub.shouldExit(0);
-  });
-}