Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/test/dart2js/omits_source_map_in_release_test.dart

-// Copyright (c) 2013, 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.
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("omits source maps from a release build", () {
-    d.dir(
-        appPath,
-        [
-            d.appPubspec(),
-            d.dir(
-                "web",
-                [d.file("main.dart", "void main() => print('hello');")])]).create();
-
-    schedulePub(
-        args: ["build"],
-        output: new RegExp(r'Built 1 file to "build".'),
-        exitCode: 0);
-
-    d.dir(
-        appPath,
-        [d.dir('build', [d.dir('web', [d.nothing('main.dart.js.map')])])]).validate();
-  });
-}