Get rid of old redundant methods in io.dart.

utils/pub/validator/name.dart

Index: utils/pub/validator/name.dart
diff --git a/utils/pub/validator/name.dart b/utils/pub/validator/name.dart
index 0386089c9821fa9c226cec98ed0338037159c4ba..1bd51346b7c6fca299a99c2b891df7a4fcc690ae 100644
--- a/utils/pub/validator/name.dart
+++ b/utils/pub/validator/name.dart
@@ -8,6 +8,7 @@ import 'dart:async';
 import 'dart:io';
 
 import '../../../pkg/path/lib/path.dart' as path;
+
 import '../entrypoint.dart';
 import '../io.dart';
 import '../utils.dart';
@@ -53,7 +54,7 @@ class NameValidator extends Validator {
       return listDir(libDir, recursive: true);
     }).then((files) {
       return files
-          .map((file) => relativeTo(file, dirname(libDir)))
+          .map((file) => path.relative(file, from: path.dirname(libDir)))
           .where((file) => !splitPath(file).contains("src") &&
                            path.extension(file) == '.dart')
           .toList();