Refactor test servers to make them less global.

test/get/hosted/stay_locked_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.
 
 import 'package:path/path.dart' as path;
 import 'package:pub/src/io.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   integration('keeps a pub server package locked to the version in the '
       'lockfile', () {
     servePackages((builder) => builder.serve("foo", "1.0.0"));
 
     d.appDir({"foo": "any"}).create();
 
     // This should lock the foo dependency to version 1.0.0.
     pubGet();
 
     d.packagesDir({"foo": "1.0.0"}).validate();
 
     // Delete the packages path to simulate a new checkout of the application.
     schedule(() => deleteEntry(path.join(sandboxDir, packagesPath)));
 
     // Start serving a newer package as well.
-    servePackages((builder) => builder.serve("foo", "1.0.1"));
+    globalPackageServer.add((builder) => builder.serve("foo", "1.0.1"));
 
     // This shouldn't upgrade the foo dependency due to the lockfile.
     pubGet();
 
     d.packagesDir({"foo": "1.0.0"}).validate();
   });
 }