Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/snapshot/snapshots_transformed_code_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 REPLACE_TRANSFORMER = """
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-class ReplaceTransformer extends Transformer {
-  ReplaceTransformer.asPlugin();
-
-  String get allowedExtensions => '.dart';
-
-  Future apply(Transform transform) {
-    return transform.primaryInput.readAsString().then((contents) {
-      transform.addOutput(new Asset.fromString(transform.primaryInput.id,
-          contents.replaceAll("REPLACE ME", "hello!")));
-    });
-  }
-}
-""";
-
-main() {
-  initConfig();
-  integration("snapshots the transformed version of an executable", () {
-    servePackages((builder) {
-      builder.serveRepoPackage('barback');
-
-      builder.serve("foo", "1.2.3", deps: {
-        "barback": "any"
-      }, pubspec: {
-        'transformers': ['foo']
-      },
-          contents: [
-              d.dir("lib", [d.file("foo.dart", REPLACE_TRANSFORMER)]),
-              d.dir("bin", [d.file("hello.dart", """
-final message = 'REPLACE ME';
-
-void main() => print(message);
-"""),])]);
-    });
-
-    d.appDir({
-      "foo": "1.2.3"
-    }).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();
-  });
-}