Refactor test servers to make them less global.

test/downgrade/unlock_dependers_test.dart

Index: test/downgrade/unlock_dependers_test.dart
diff --git a/test/downgrade/unlock_dependers_test.dart b/test/downgrade/unlock_dependers_test.dart
index 70a628215ee4e6edaa91b6249d657cc2a06fb33c..6a644c64a8742eeeaf6bd3cea98c4cc410540918 100644
--- a/test/downgrade/unlock_dependers_test.dart
+++ b/test/downgrade/unlock_dependers_test.dart
@@ -22,7 +22,7 @@ main() {
       "bar": "2.0.0"
     }).validate();
 
-    servePackages((builder) {
+    globalPackageServer.add((builder) {
       builder.serve("foo", "1.0.0", deps: {"bar": "any"});
       builder.serve("bar", "1.0.0");
     });