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

sdk/lib/_internal/pub_generated/test/global/activate/supports_version_solver_backtracking_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';
-
-main() {
-  initConfig();
-  integration('performs verison solver backtracking if necessary', () {
-    servePackages((builder) {
-      builder.serve("foo", "1.1.0", pubspec: {
-        "environment": {
-          "sdk": ">=0.1.2 <0.2.0"
-        }
-      });
-      builder.serve("foo", "1.2.0", pubspec: {
-        "environment": {
-          "sdk": ">=0.1.3 <0.2.0"
-        }
-      });
-    });
-
-    schedulePub(args: ["global", "activate", "foo"]);
-
-    // foo 1.2.0 won't be picked because its SDK constraint conflicts with the
-    // dummy SDK version 0.1.2+3.
-    d.dir(
-        cachePath,
-        [
-            d.dir(
-                'global_packages',
-                [
-                    d.dir('foo', [d.matcherFile('pubspec.lock', contains('1.1.0'))])])]).validate();
-  });
-}