Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/get/switch_source_test.dart

-// Copyright (c) 2012, 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 d.file.
-
-library pub_tests;
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('re-gets a package if its source has changed', () {
-    servePackages((builder) => builder.serve("foo", "1.2.3"));
-
-    d.dir(
-        'foo',
-        [d.libDir('foo', 'foo 0.0.1'), d.libPubspec('foo', '0.0.1')]).create();
-
-    d.appDir({
-      "foo": {
-        "path": "../foo"
-      }
-    }).create();
-
-    pubGet();
-
-    d.packagesDir({
-      "foo": "0.0.1"
-    }).validate();
-    d.appDir({
-      "foo": "any"
-    }).create();
-
-    pubGet();
-
-    d.packagesDir({
-      "foo": "1.2.3"
-    }).validate();
-  });
-}