Make pub strong-mode clean.

lib/src/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.
 
 import 'dart:async';
 
 import 'entrypoint.dart';
 import 'log.dart' as log;
 import 'utils.dart';
 import 'validator/compiled_dartdoc.dart';
@@ skipping 57 matching lines @@
       new CompiledDartdocValidator(entrypoint),
       new Utf8ReadmeValidator(entrypoint),
       new SdkConstraintValidator(entrypoint)
     ];
     if (packageSize != null) {
       validators.add(new SizeValidator(entrypoint, packageSize));
     }
 
     return Future.wait(validators.map((validator) => validator.validate()))
       .then((_) {
-      var errors =
-          flatten(validators.map((validator) => validator.errors));
-      var warnings =
-          flatten(validators.map((validator) => validator.warnings));
+      var errors = validators.expand((validator) => validator.errors);
+      var warnings = validators.expand((validator) => validator.warnings);
 
       if (!errors.isEmpty) {
         log.error("Missing requirements:");
         for (var error in errors) {
           log.error("* ${error.split('\n').join('\n  ')}");
         }
         log.error("");
       }
 
       if (!warnings.isEmpty) {
         log.warning("Suggestions:");
         for (var warning in warnings) {
           log.warning("* ${warning.split('\n').join('\n  ')}");
         }
         log.warning("");
       }
 
       return new Pair<List<String>, List<String>>(errors, warnings);
     });
   }
 }