Refactor test servers to make them less global.

test/get/hosted/unlock_if_version_doesnt_exist_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 'package:path/path.dart' as p;
 import 'package:pub/src/io.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   integration('upgrades a locked pub server package with a nonexistent version',
       () {
     servePackages((builder) => builder.serve("foo", "1.0.0"));
 
     d.appDir({"foo": "any"}).create();
     pubGet();
     d.packagesDir({"foo": "1.0.0"}).validate();
 
     schedule(() => deleteEntry(p.join(sandboxDir, cachePath)));
 
-    servePackages((builder) => builder.serve("foo", "1.0.1"), replace: true);
+    globalPackageServer.replace((builder) => builder.serve("foo", "1.0.1"));
     pubGet();
     d.packagesDir({"foo": "1.0.1"}).validate();
   });
 }