GN: Don't depend on dart_bootstrap if there is a usable prebuilt sdk

tools/dart_for_gn.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.
-
-# Usage: dart_for_gn.py path_to_dart_binary [arguments]
-#
-# We have gn set up so that actions can only run python scripts.
-# We use this wrapper script when we need an action to run the Dart VM
-# The first command line argument is mandatory and should be the absolute path
-# to the Dart VM binary.
-
-import subprocess
-import sys
-
-def run_command(command):
-  try:
-    subprocess.check_output(command, stderr=subprocess.STDOUT)
-    return 0
-  except subprocess.CalledProcessError as e:
-    return ("Command failed: " + ' '.join(command) + "\n" +
-            "output: " + e.output)
-
-def main(argv):
-  if len(argv) < 2:
-    print "Requires path to Dart VM binary as first argument"
-    return -1
-  result = run_command(argv[1:])
-  if result != 0:
-    print result
-    return -1
-  return 0
-
-if __name__ == '__main__':
-    sys.exit(main(sys.argv))