Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/test/run/runs_transformer_in_entrypoint_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';
-import '../serve/utils.dart';
-
-const SCRIPT = """
-const TOKEN = "hi";
-main() {
-  print(TOKEN);
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration('runs transformers in the entrypoint package', () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir(
-                "lib",
-                [d.dir("src", [d.file("transformer.dart", dartTransformer("transformed"))])]),
-            d.dir("bin", [d.file("hi.dart", SCRIPT)])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      var pub = pubRun(args: ["hi"]);
-
-      pub.stdout.expect("(hi, transformed)");
-      pub.shouldExit();
-    });
-  });
-}