Revert "GN: Make GN the default"

tools/bots/gn_build.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
 
 SCRIPT_DIR = os.path.dirname(sys.argv[0])
 DART_ROOT = os.path.realpath(os.path.join(SCRIPT_DIR, '..', '..'))
 
 def main(argv):
-  os.environ["DART_USE_GYP"] = "1"
+  os.environ["DART_USE_GN"] = "1"
   generate_buildfiles = os.path.join(
       DART_ROOT, 'tools', 'generate_buildfiles.py')
   gclient_result = subprocess.call(['python', generate_buildfiles])
   if gclient_result != 0:
     return gclient_result
 
   build_py = os.path.join(DART_ROOT, 'tools', 'build.py')
   build_result = subprocess.call(['python', build_py] + argv[1:])
   if build_result != 0:
     return build_result
   return 0
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))