Fix Debian builder.

tools/patch_sdk.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 argparse
 import os
 import subprocess
 import sys
 import utils
 
+usage = """patch_sdk.py [options]"""
+
+def BuildArguments():
+  result = argparse.ArgumentParser(usage=usage)
+  result.add_argument("--dart-executable", help="dart executable", default=None)
+  return result
+
 def main():
-  dart = os.path.join(utils.CheckedInSdkPath(), 'bin', 'dart')
+  # Parse the options.
+  parser = BuildArguments()
+  (options, args) = parser.parse_known_args()
+  if options.dart_executable is not None:
+    options.dart_executable = os.path.abspath(options.dart_executable)
+  else:
+    options.dart_executable = os.path.join(utils.CheckedInSdkPath(), 'bin', 'dart')
   dart_file = os.path.join(os.path.dirname(__file__), 'patch_sdk.dart')
-  subprocess.check_call([dart, dart_file] + sys.argv[1:]);
+  subprocess.check_call([options.dart_executable, dart_file] + args);
 
 if __name__ == '__main__':
   main()