Add default tar file name for source tarball

tools/create_tarball.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2014, 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.
 #
 
 # Script to build a tarball of the Dart source.
 #
 # The tarball includes all the source needed to build Dart. This
@@ skipping 137 matching lines @@
     print 'Tarball can only be created on linux'
     return -1
 
   # Parse the options.
   parser = BuildOptions()
   (options, args) = parser.parse_args()
   if options.verbose:
     global verbose
     verbose = True
 
-  if not options.tar_filename:
-    raise Exception('Please specify an output filename')
+  tar_filename = options.tar_filename
+  if not tar_filename:
+    tar_filename = join(DART_DIR,
+                        utils.GetBuildDir(HOST_OS, HOST_OS),
+                        'dart-%s.tar.gz' % utils.GetVersion())
 
-  CreateTarball(options.tar_filename)
+  CreateTarball(tar_filename)
 
 if __name__ == '__main__':
   sys.exit(Main())