Don't hardcode "dart" executable name.

tools/bots/ddc_tests.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2016, 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 os.path
 import shutil
 import sys
 import subprocess
 
 import bot
 import bot_utils
 
 utils = bot_utils.GetUtils()
 
 BUILD_OS = utils.GuessOS()
 
 (bot_name, _) = bot.GetBotName()
 CHANNEL = bot_utils.GetChannelFromName(bot_name)
 
 if __name__ == '__main__':
   with utils.ChangedWorkingDirectory('pkg/dev_compiler'):
     with bot.BuildStep('npm install'):
       bot.RunProcess(['npm', 'install'])
 
     with bot.BuildStep('Compile tests and run unit tests'):
-      bot.RunProcess(['dart', 'test/all_tests.dart'])
+      bot.RunProcess([utils.CheckedInSdkExecutable(), 'test/all_tests.dart'])
 
     with bot.BuildStep('Execute compiled tests'):
       bot.RunProcess(['npm', 'test'])