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

tools/test.py

Index: tools/test.py
diff --git a/tools/test.py b/tools/test.py
index bd16393b0af13665a6bafb50512a5d1eb22befba..3ab247b4c4c552c51d2a41c25417c37deaafa4db 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -15,11 +15,6 @@ 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'):
@@ -27,7 +22,7 @@ def Main():
   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