Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/global/activate/different_version_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 '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration(
-      "discards the previous active version if it doesn't match the " "constraint",
-      () {
-    servePackages((builder) {
-      builder.serve("foo", "1.0.0");
-      builder.serve("foo", "2.0.0");
-    });
-
-    // Activate 1.0.0.
-    schedulePub(args: ["global", "activate", "foo", "1.0.0"]);
-
-    // Activating it again with a different constraint changes the version.
-    schedulePub(args: ["global", "activate", "foo", ">1.0.0"], output: """
-        Package foo is currently active at version 1.0.0.
-        Resolving dependencies...
-        + foo 2.0.0
-        Downloading foo 2.0.0...
-        Precompiling executables...
-        Loading source assets...
-        Activated foo 2.0.0.""");
-  });
-}