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

utils/pub/validator.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library validator;
 
 import 'entrypoint.dart';
 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';
 
 /// The base class for validators that check whether a package is fit for
 /// uploading. Each validator should override [errors], [warnings], or both to
 /// return lists of errors or warnings to display to the user. Errors will cause
 /// the package not to be uploaded; warnings will require the user to confirm
 /// the upload.
 abstract class Validator {
@@ skipping 11 matching lines @@
   /// Validates the entrypoint, adding any errors and warnings to [errors] and
   /// [warnings], respectively.
   Future validate();
 
   /// Run all validators on the [entrypoint] package and print their results.
   /// The future will complete with the error and warning messages,
   /// respectively.
   static Future<Pair<List<String>, List<String>>> runAll(
       Entrypoint entrypoint) {
     var validators = [
+      new LibValidator(entrypoint),
       new LicenseValidator(entrypoint),
       new NameValidator(entrypoint),
       new PubspecFieldValidator(entrypoint)
     ];
 
     // TODO(nweiz): The sleep 0 here forces us to go async. This works around
     // 3356, which causes a bug if all validators are (synchronously) using
     // Future.immediate and an error is thrown before a handler is set up.
     return sleep(0).chain((_) {
       return Futures.wait(validators.map((validator) => validator.validate()));
@@ skipping 14 matching lines @@
         for (var warning in warnings) {
           log.warning("* $warning");
         }
         log.warning("");
       }
 
       return new Pair<List<String>, List<String>>(errors, warnings);
     });
   }
 }