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

tools/generate_buildfiles.py

Index: tools/generate_buildfiles.py
diff --git a/tools/generate_buildfiles.py b/tools/generate_buildfiles.py
index 391a15a07ea4358d84c0ed8783ebdaa8b4f7b0b2..9aa26e725881842e497fc6e87155c26c509bdfea 100644
--- a/tools/generate_buildfiles.py
+++ b/tools/generate_buildfiles.py
@@ -12,12 +12,17 @@ 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()
@@ -68,6 +73,9 @@ def parse_args(args):
 
 
 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()