GN: Fall back on dart_bootstrap where appropriate.

tools/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.
 
 import argparse
 import multiprocessing
 import os
 import subprocess
 import sys
@@ skipping 54 matching lines @@
   host_os = host_os_for_gn(HOST_OS)
   if target_os == 'host':
     gn_args['target_os'] = host_os
   else:
     gn_args['target_os'] = target_os
 
   gn_args['dart_target_arch'] = arch
   gn_args['target_cpu'] = target_cpu_for_arch(arch, target_os)
   gn_args['host_cpu'] = host_cpu_for_arch(arch)
 
-  # TODO(zra): This is for the observatory, which currently builds using the
-  # checked-in sdk. If/when the observatory no longer builds with the
-  # checked-in sdk, this can be removed.
-  pub = 'pub'
-  if host_os == 'win':
-    pub = pub + ".bat"
-  gn_args['dart_host_pub_exe'] = os.path.join(
-      DART_ROOT, 'tools', 'sdks', host_os, 'dart-sdk', 'bin', pub)
+  # See: runtime/observatory/BUILD.gn.
+  # This allows the standalone build of the observatory to fall back on
+  # dart_bootstrap if the prebuilt SDK doesn't work.
+  gn_args['dart_host_pub_exe'] = ""
 
   # For Fuchsia support, the default is to not compile in the root
   # certificates.
   gn_args['dart_use_fallback_root_certificates'] = True
 
   gn_args['dart_zlib_path'] = "//runtime/bin/zlib"
 
   # Use tcmalloc only when targeting Linux and when not using ASAN.
   gn_args['dart_use_tcmalloc'] = (gn_args['target_os'] == 'linux'
                                   and not args.asan)
@@ skipping 223 matching lines @@
       return 1
 
   endtime = time.time()
   if args.verbose:
     print ("GN Time: %.3f seconds" % (endtime - starttime))
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))