Fix dart_pkg.py build crash

mojo/public/tools/dart_pkg.py

 #!/usr/bin/env python
 #
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Utility for dart_pkg and dart_pkg_app rules"""
 
 import argparse
 import errno
@@ skipping 30 matching lines @@
 
 
 def mojom_filter(path):
     if os.path.isdir(path):
         return True
     _, ext = os.path.splitext(path)
     return ext == '.mojom'
 
 
 def ensure_dir_exists(path):
-    if not os.path.exists(path):
-        os.makedirs(path)
+    abspath = os.path.abspath(path)
+    if not os.path.exists(abspath):
+        os.makedirs(abspath)
 
 
 def has_pubspec_yaml(paths):
     for path in paths:
         _, filename = os.path.split(path)
         if 'pubspec.yaml' == filename:
             return True
     return False
 
 
@@ skipping 203 matching lines @@
     # Remove any broken symlinks in target_dir and package root.
     remove_broken_symlinks(target_dir)
     remove_broken_symlinks(args.package_root)
 
     # Write stamp file.
     with open(args.stamp_file, 'w'):
         pass
 
 if __name__ == '__main__':
     sys.exit(main())