Adds an environment variable that makes generate_buildfiles.py a no-op

tools/generate_buildfiles.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import argparse
 import os
 import subprocess
 import sys
 import utils
 
 SCRIPT_DIR = os.path.dirname(sys.argv[0])
 DART_ROOT = os.path.realpath(os.path.join(SCRIPT_DIR, '..'))
 DART_USE_GN = "DART_USE_GN"
+DART_DISABLE_BUILDFILES = "DART_DISABLE_BUILDFILES"
 
 
 def use_gn():
   return DART_USE_GN in os.environ
 
 
+def disable_buildfiles():
+  return DART_DISABLE_BUILDFILES in os.environ
+
+
 def execute(args):
   process = subprocess.Popen(args, cwd=DART_ROOT)
   process.wait()
   return process.returncode
 
 
 def run_gn():
   gn_command = [
     'python',
     os.path.join(DART_ROOT, 'tools', 'gn.py'),
@@ skipping 30 matching lines @@
       action='store_true')
 
   options = parser.parse_args(args)
   # If gn is enabled one way or another, then disable gyp
   if options.gn:
     options.gyp = False
   return options
 
 
 def main(argv):
+  # Check the environment and become a no-op if directed.
+  if disable_buildfiles():
+    return 0
   options = parse_args(argv)
   if options.gn:
     return run_gn()
   else:
     return run_gyp()
 
 
 if __name__ == '__main__':
     sys.exit(main(sys.argv))