Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/upgrade/hosted/upgrade_removed_constraints_test.dart

-// Copyright (c) 2012, 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.
-
-library pub_tests;
-
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("upgrades dependencies whose constraints have been removed", () {
-    servePackages((builder) {
-      builder.serve("foo", "1.0.0", deps: {
-        "shared-dep": "any"
-      });
-      builder.serve("bar", "1.0.0", deps: {
-        "shared-dep": "<2.0.0"
-      });
-      builder.serve("shared-dep", "1.0.0");
-      builder.serve("shared-dep", "2.0.0");
-    });
-
-    d.appDir({
-      "foo": "any",
-      "bar": "any"
-    }).create();
-
-    pubUpgrade();
-
-    d.packagesDir({
-      "foo": "1.0.0",
-      "bar": "1.0.0",
-      "shared-dep": "1.0.0"
-    }).validate();
-
-    d.appDir({
-      "foo": "any"
-    }).create();
-
-    pubUpgrade();
-
-    d.packagesDir({
-      "foo": "1.0.0",
-      "bar": null,
-      "shared-dep": "2.0.0"
-    }).validate();
-  });
-}