Refactor test servers to make them less global.

test/get/cache_transformed_dependency_test.dart

Index: test/get/cache_transformed_dependency_test.dart
diff --git a/test/get/cache_transformed_dependency_test.dart b/test/get/cache_transformed_dependency_test.dart
index b0d64e1aeb5f2fed0a211e0b6c479b6706094ef9..a1ab8c440d90e361e319a8ca067866f91413f02f 100644
--- a/test/get/cache_transformed_dependency_test.dart
+++ b/test/get/cache_transformed_dependency_test.dart
@@ -203,7 +203,7 @@ main() {
     d.appDir({"foo": "1.2.3"}).create();
     pubGet(output: contains("Precompiled foo."));
 
-    servePackages((builder) => builder.serve("bar", "6.0.0"));
+    globalPackageServer.add((builder) => builder.serve("bar", "6.0.0"));
     pubUpgrade(output: contains("Precompiled foo."));
   });
 
@@ -227,7 +227,7 @@ main() {
     d.appDir({"foo": "1.2.3"}).create();
     pubGet(output: contains("Precompiled foo."));
 
-    servePackages((builder) => builder.serve("bar", "6.0.0"));
+    globalPackageServer.add((builder) => builder.serve("bar", "6.0.0"));
     pubUpgrade(output: isNot(contains("Precompiled foo.")));
   });