Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/dart2js/outputs_deferred_libraries_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 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-const MAIN = """
-import 'dart:async';
-
-import 'a.dart' deferred as a;
-import 'b.dart' deferred as b;
-
-void main() {
-  Future.wait([lazyA.loadLibrary(), lazyB.loadLibrary()]).then((_) {
-    a.fn();
-    b.fn();
-  });
-}
-""";
-
-const A = """
-library a;
-
-fn() => print("a");
-""";
-
-const B = """
-library b;
-
-fn() => print("b");
-""";
-
-main() {
-  initConfig();
-  integration("compiles deferred libraries to separate outputs", () {
-    // Dart2js can take a long time to compile dart code, so we increase the
-    // timeout to cope with that.
-    currentSchedule.timeout *= 3;
-
-    d.dir(
-        appPath,
-        [
-            d.appPubspec(),
-            d.dir(
-                'web',
-                [
-                    d.file('main.dart', MAIN),
-                    d.file('a.dart', A),
-                    d.file('b.dart', B)])]).create();
-
-    schedulePub(
-        args: ["build"],
-        output: new RegExp(r'Built 3 files to "build".'));
-
-    d.dir(
-        appPath,
-        [
-            d.dir(
-                'build',
-                [
-                    d.dir(
-                        'web',
-                        [
-                            d.matcherFile('main.dart.js', isNot(isEmpty)),
-                            d.matcherFile('main.dart.js_1.part.js', isNot(isEmpty)),
-                            d.matcherFile('main.dart.js_2.part.js', isNot(isEmpty)),])])]).validate();
-  });
-}