Add a validator for top-level directory names.

utils/pub/validator.dart

Index: utils/pub/validator.dart
diff --git a/utils/pub/validator.dart b/utils/pub/validator.dart
index 48f7768dd09c5e77fef39a325f08a98ef3b6b544..63a2386c07290ae3525efe93bc063900ce7e1e0c 100644
--- a/utils/pub/validator.dart
+++ b/utils/pub/validator.dart
@@ -10,6 +10,7 @@ import 'io.dart';
 import 'system_cache.dart';
 import 'utils.dart';
 import 'validator/dependency.dart';
+import 'validator/directory.dart';
 import 'validator/lib.dart';
 import 'validator/license.dart';
 import 'validator/name.dart';
@@ -46,7 +47,8 @@ abstract class Validator {
       new LicenseValidator(entrypoint),
       new NameValidator(entrypoint),
       new PubspecFieldValidator(entrypoint),
-      new DependencyValidator(entrypoint)
+      new DependencyValidator(entrypoint),
+      new DirectoryValidator(entrypoint)
     ];
 
     // TODO(nweiz): The sleep 0 here forces us to go async. This works around