Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/get/hosted/resolve_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('resolves version constraints from a pub server', () {
-    servePackages((builder) {
-      builder.serve("foo", "1.2.3", deps: {
-        "baz": ">=2.0.0"
-      });
-      builder.serve("bar", "2.3.4", deps: {
-        "baz": "<3.0.0"
-      });
-      builder.serve("baz", "2.0.3");
-      builder.serve("baz", "2.0.4");
-      builder.serve("baz", "3.0.1");
-    });
-
-    d.appDir({
-      "foo": "any",
-      "bar": "any"
-    }).create();
-
-    pubGet();
-
-    d.cacheDir({
-      "foo": "1.2.3",
-      "bar": "2.3.4",
-      "baz": "2.0.4"
-    }).validate();
-
-    d.packagesDir({
-      "foo": "1.2.3",
-      "bar": "2.3.4",
-      "baz": "2.0.4"
-    }).validate();
-  });
-}