Refactor test servers to make them less global.

test/get/hosted/unlock_if_incompatible_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 '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   integration('upgrades a locked pub server package with a new incompatible '
       'constraint', () {
     servePackages((builder) => builder.serve("foo", "1.0.0"));
 
     d.appDir({"foo": "any"}).create();
 
     pubGet();
 
     d.packagesDir({"foo": "1.0.0"}).validate();
-    servePackages((builder) => builder.serve("foo", "1.0.1"));
+    globalPackageServer.add((builder) => builder.serve("foo", "1.0.1"));
     d.appDir({"foo": ">1.0.0"}).create();
 
     pubGet();
 
     d.packagesDir({"foo": "1.0.1"}).validate();
   });
 }