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

utils/pub/validator.dart

Index: utils/pub/validator.dart
diff --git a/utils/pub/validator.dart b/utils/pub/validator.dart
index dea568fd5300fea86b3537cf7e78c96b48cb7680..3e491a93a3954bc935116af4c4ded23c436ebb43 100644
--- a/utils/pub/validator.dart
+++ b/utils/pub/validator.dart
@@ -9,6 +9,7 @@ import 'log.dart' as log;
 import 'io.dart';
 import 'system_cache.dart';
 import 'utils.dart';
+import 'validator/lib.dart';
 import 'validator/license.dart';
 import 'validator/name.dart';
 import 'validator/pubspec_field.dart';
@@ -40,6 +41,7 @@ abstract class Validator {
   static Future<Pair<List<String>, List<String>>> runAll(
       Entrypoint entrypoint) {
     var validators = [
+      new LibValidator(entrypoint),
       new LicenseValidator(entrypoint),
       new NameValidator(entrypoint),
       new PubspecFieldValidator(entrypoint)