Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/build/cleans_entire_build_directory_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-
-  integration("cleans entire build directory before a build", () {
-    d.dir(
-        appPath,
-        [
-            d.appPubspec(),
-            d.dir('example', [d.file('file.txt', 'example')]),
-            d.dir('test', [d.file('file.txt', 'test')])]).create();
-
-    // Make a build directory containing "example".
-    schedulePub(
-        args: ["build", "example"],
-        output: new RegExp(r'Built 1 file to "build".'));
-
-    // Now build again with just "test". Should wipe out "example".
-    schedulePub(
-        args: ["build", "test"],
-        output: new RegExp(r'Built 1 file to "build".'));
-
-    d.dir(
-        appPath,
-        [
-            d.dir(
-                'build',
-                [
-                    d.nothing('example'),
-                    d.dir('test', [d.file('file.txt', 'test')]),])]).validate();
-  });
-}