Refactor test servers to make them less global.

test/upgrade/hosted/unlock_if_necessary_test.dart

Index: test/upgrade/hosted/unlock_if_necessary_test.dart
diff --git a/test/upgrade/hosted/unlock_if_necessary_test.dart b/test/upgrade/hosted/unlock_if_necessary_test.dart
index ed38adf120863434a24875fd903cc28fb63c5fe0..1b015070607ba86f47ccba024f423f49bf5e0af5 100644
--- a/test/upgrade/hosted/unlock_if_necessary_test.dart
+++ b/test/upgrade/hosted/unlock_if_necessary_test.dart
@@ -22,7 +22,7 @@ main() {
       "foo_dep": "1.0.0"
     }).validate();
 
-    servePackages((builder) {
+    globalPackageServer.add((builder) {
       builder.serve("foo", "2.0.0", deps: {"foo_dep": ">1.0.0"});
       builder.serve("foo_dep", "2.0.0");
     });