Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/transformer/asset_not_found_exceptions_are_detectable_test.dart

-// Copyright (c) 2014, 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.
-
-library pub_tests;
-
-import 'dart:convert';
-
-import 'package:scheduled_test/scheduled_stream.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-import '../serve/utils.dart';
-
-final transformer = """
-import 'dart:async';
-import 'dart:convert';
-
-import 'package:barback/barback.dart';
-
-class GetInputTransformer extends Transformer {
-  GetInputTransformer.asPlugin();
-
-  String get allowedExtensions => '.txt';
-
-  Future apply(Transform transform) {
-    return transform.readInputAsString(new AssetId('myapp', 'nonexistent'))
-        .catchError((error) {
-      if (error is! AssetNotFoundException) throw error;
-      transform.addOutput(new Asset.fromString(transform.primaryInput.id,
-          JSON.encode({
-        'package': error.id.package,
-        'path': error.id.path
-      })));
-    });
-  }
-}
-""";
-
-main() {
-  initConfig();
-  withBarbackVersions("any", () {
-    integration("AssetNotFoundExceptions are detectable", () {
-      d.dir(appPath, [d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-            d.dir("lib", [d.dir("src", [d.file("transformer.dart", transformer)])]),
-            d.dir("web", [d.file("foo.txt", "foo")])]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      var server = pubServe();
-      requestShouldSucceed("foo.txt", JSON.encode({
-        "package": "myapp",
-        "path": "nonexistent"
-      }));
-      endPubServe();
-
-      // Since the AssetNotFoundException was caught and handled, the server
-      // shouldn't print any error information for it.
-      server.stderr.expect(isDone);
-    });
-  });
-}