Add a missing dart:io import to pub.

R=rnystrom@google.com
TBR

Review URL: https://codereview.chromium.org//332003003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37528 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/package.dart

@@ -4,6 +4,8 @@
 
 library pub.package;
 
+import 'dart:io';
+
 import 'package:path/path.dart' as path;
 
 import 'io.dart';