Refactor test servers to make them less global.

test/snapshot/upgrades_snapshot_for_dependency_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:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
@@ skipping 17 matching lines @@
     });
 
     d.appDir({"foo": "any"}).create();
 
     pubGet(output: contains("Precompiled foo:hello."));
 
     d.dir(p.join(appPath, '.pub', 'bin', 'foo'), [
       d.matcherFile('hello.dart.snapshot', contains('hello!'))
     ]).validate();
 
-    servePackages((builder) {
+    globalPackageServer.add((builder) {
       builder.serve("bar", "1.2.4", contents: [
         d.dir("lib", [d.file("bar.dart", "final message = 'hello 2!';")]),
       ]);
     });
 
     pubUpgrade(output: contains("Precompiled foo:hello."));
 
     d.dir(p.join(appPath, '.pub', 'bin', 'foo'), [
       d.matcherFile('hello.dart.snapshot', contains('hello 2!'))
     ]).validate();
 
     var process = pubRun(args: ['foo:hello']);
     process.stdout.expect("hello 2!");
     process.shouldExit();
   });
 }