Refactor test servers to make them less global.

test/snapshot/upgrades_snapshot_for_dependency_test.dart

Index: test/snapshot/upgrades_snapshot_for_dependency_test.dart
diff --git a/test/snapshot/upgrades_snapshot_for_dependency_test.dart b/test/snapshot/upgrades_snapshot_for_dependency_test.dart
index 5ec2d4439b056d1f0a537afaf25e1550b6d46906..aec4b51b4acb4dcb3820589fd1db47c94c04badf 100644
--- a/test/snapshot/upgrades_snapshot_for_dependency_test.dart
+++ b/test/snapshot/upgrades_snapshot_for_dependency_test.dart
@@ -35,7 +35,7 @@ void main() => print(message);
       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!';")]),
       ]);