Rename .sdkext to _sdkext and fix sky_tool

sky/sdk/lib/sky_tool

 #!/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.
 
 import argparse
 import json
 import logging
 import os
 import re
@@ skipping 117 matching lines @@
         return subprocess.check_output(pm_path_cmd).strip() != ''
 
     def run(self, args, pids):
         StopSky().run(args, pids)
 
         project_or_path = os.path.abspath(args.project_or_path)
 
         if os.path.isdir(project_or_path):
             sky_server_root = project_or_path
             main_dart = os.path.join(project_or_path, 'lib', 'main.dart')
-            missing_msg = "Missing lib/main.dart in project: %s" % project_dir
+            missing_msg = "Missing lib/main.dart in project: %s" % project_or_path
         else:
             # FIXME: This assumes the path is at the root of the project!
             # Instead we should walk up looking for a pubspec.yaml
             sky_server_root = os.path.dirname(project_or_path)
             main_dart = project_or_path
             missing_msg = "%s does not exist." % main_dart
 
         if not os.path.isfile(main_dart):
             print missing_msg
             return 2
@@ skipping 159 matching lines @@
         args = parser.parse_args()
         pids = Pids.read_from(PID_FILE_PATH, PID_FILE_KEYS)
         exit_code = args.func(args, pids)
         # We could do this with an at-exit handler instead?
         pids.write_to(PID_FILE_PATH)
         sys.exit(exit_code)
 
 
 if __name__ == '__main__':
     SkyShellRunner().main()