Move a helper function to find out Dart binary into utils.py

tools/test.py

 #!/usr/bin/env python
 # Copyright (c) 2011, 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.
 
 import os
 import platform
 import string
 import subprocess
 import sys
 
 import utils
 
 
 def Main():
   args = sys.argv[1:]
   tools_dir = os.path.dirname(os.path.realpath(__file__))
-  dart_binary_prefix = os.path.join(tools_dir, 'testing', 'bin')
-  if utils.IsWindows():
-    dart_binary = os.path.join(dart_binary_prefix, 'windows', 'dart.exe')
-  else:
-    dart_binary = os.path.join(dart_binary_prefix, utils.GuessOS(), 'dart')
   current_directory = os.path.abspath('');
   client = os.path.abspath(os.path.join(tools_dir, '..'));
   if current_directory == os.path.join(client, 'runtime'):
     dart_script_name = 'test-runtime.dart'
   else:
     dart_script_name = 'test.dart'
   dart_test_script = string.join([tools_dir, dart_script_name], os.sep)
-  command = [dart_binary, dart_test_script] + args
+  command = [utils.DartBinary(), dart_test_script] + args
   exit_code = subprocess.call(command)
   utils.DiagnoseExitCode(exit_code, command)
   return exit_code
 
 
 if __name__ == '__main__':
   sys.exit(Main())