Add error and message to tools/make_links.py for duplicate packages.

tools/make_links.py

 #!/usr/bin/env python
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # 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.
 
 
 """Tool for creating symlinks from SOURCES to TARGET.
 
 For each SOURCE in SOURCES create a link from SOURCE to TARGET.  If a
 SOURCE ends with .../lib, the lib suffix is ignored when determining
@@ skipping 10 matching lines @@
 
 """
 
 import optparse
 import os
 import shutil
 import subprocess
 import sys
 import utils
 
+# Useful messages when we find orphaned checkouts.
+old_directories = {'package_config':
+  'Please remove third_party/pkg/package_config.'}
 
 def get_options():
   result = optparse.OptionParser()
   result.add_option("--timestamp_file", "",
       help='Create a timestamp file when done creating the links.',
       default='')
   return result.parse_args()
 
 def make_link(source, target, orig_source):
   if os.path.islink(target):
@@ skipping 38 matching lines @@
     for link in os.listdir(target):
       full_link = os.path.join(target, link)
       if os.path.isdir(full_link) and utils.IsWindows():
         # It seems like python on Windows is treating pseudo symlinks to
         # directories as directories.
         os.rmdir(full_link)
       else:
         os.remove(full_link)
   else:
     os.makedirs(target)
+  linked_names = {}; 
   for source in args[1:]:
     # Assume the source directory is named ".../NAME/lib".
     split = source.split(':')
     name = None
     if len(split) == 2: (source, name) = split
 
     (path, lib) = os.path.split(source)
     if lib != 'lib':
       path = source
     # Remove any additional path components preceding NAME, if one wasn't
     # specified explicitly.
     if not name: (_, name) = os.path.split(path)
+    # We have an issue with left-behind checkouts in third_party/pkg and
+    # third_party/pkg_tested when we move entries in DEPS. This reports them.
+    if name in linked_names:
+      print 'Duplicate directory %s is linked to both %s and %s.' % (
+          name, linked_names[name], path)
+      if name in old_directories:
+        print old_directories[name]
+      return 1
+    linked_names[name] = path
     orig_source = source
     if utils.GuessOS() == 'win32':
       source = os.path.relpath(source)
     else:
       source = os.path.relpath(source, start=target)
     exit_code = make_link(source, os.path.join(target, name), orig_source)
     if exit_code != 0:
       return exit_code
   create_timestamp_file(options)
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))