Add a validator that checks that the lib directory is not empty.

utils/tests/pub/oauth2_test.dart

Index: utils/tests/pub/oauth2_test.dart
diff --git a/utils/tests/pub/oauth2_test.dart b/utils/tests/pub/oauth2_test.dart
index fdbf92bd341d2378e11dd2be7be2065890d718a0..32e4a75a729b647d628df36057dccc226a2699a5 100644
--- a/utils/tests/pub/oauth2_test.dart
+++ b/utils/tests/pub/oauth2_test.dart
@@ -15,12 +15,7 @@ import '../../pub/io.dart';
 import '../../pub/utils.dart';
 
 main() {
-  setUp(() {
-    dir(appPath, [
-      libPubspec("test_pkg", "1.0.0"),
-      file("LICENSE", "Eh, do what you want.")
-    ]).scheduleCreate();
-  });
+  setUp(() => normalPackage.scheduleCreate());
 
   test('with no credentials.json, authenticates and saves credentials.json',
       () {