Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/snapshot/doesnt_load_irrelevant_transformers_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.
-
-library pub_tests;
-
-import 'package:path/path.dart' as p;
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-const BROKEN_TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class BrokenTransformer extends Transformer {
-  BrokenTransformer.asPlugin();
-
-  // This file intentionally has a syntax error so that any attempt to load it
-  // will crash.
-""";
-
-main() {
-  initConfig();
-
-  // Regression test for issue 20917.
-  integration("snapshots the transformed version of an executable", () {
-    servePackages((builder) {
-      builder.serveRepoPackage('barback');
-
-      builder.serve(
-          "foo",
-          "1.2.3",
-          contents: [
-              d.dir("bin", [d.file("hello.dart", "void main() => print('hello!');")])]);
-    });
-
-    d.dir(appPath, [d.pubspec({
-        "name": "myapp",
-        "dependencies": {
-          "foo": "1.2.3",
-          "barback": "any"
-        },
-        "transformers": ["myapp"]
-      }),
-          d.dir("lib", [d.file("transformer.dart", BROKEN_TRANSFORMER)])]).create();
-
-    pubGet(output: contains("Precompiled foo:hello."));
-
-    d.dir(
-        p.join(appPath, '.pub', 'bin'),
-        [
-            d.dir(
-                'foo',
-                [d.matcherFile('hello.dart.snapshot', contains('hello!'))])]).validate();
-
-    var process = pubRun(args: ['foo:hello']);
-    process.stdout.expect("hello!");
-    process.shouldExit();
-  });
-}