Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/dart2js/does_not_support_invalid_environment_type_test.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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';
-
-main() {
-  initConfig();
-  integration("doesn't support invalid environment type", () {
-    d.dir(appPath, [d.pubspec({
-        "name": "myapp",
-        "transformers": [{
-            "\$dart2js": {
-              "environment": "foo",
-            }
-          }]
-      }), d.dir("web", [d.file("main.dart", "void main() {}")])]).create();
-
-    var server = pubServe();
-    // Make a request first to trigger compilation.
-    requestShould404("main.dart.js");
-    server.stderr.expect(
-        emitsLines(
-            'Build error:\n' 'Transform Dart2JS on myapp|web/main.dart threw error: '
-                'Invalid value for \$dart2js.environment: "foo" '
-                '(expected map from strings to strings).'));
-    endPubServe();
-  });
-}