add back dart:io import to pubspec.dart (#1711)

Author: jakemac53

lib/src/pubspec.dart

@@ -2,6 +2,8 @@
 // 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:io';
+
 import 'package:collection/collection.dart';
 import 'package:path/path.dart' as path;
 import 'package:pub_semver/pub_semver.dart';