Enable GN build on Mac.

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 subprocess
 import sys
 import os
 import utils
@@ skipping 30 matching lines @@
     return 'mipsel'
   if arch == 'simdbc':
     return 'arm' if os == 'android' else 'x86'
   if arch == 'simdbc64':
     return 'arm64' if os == 'android' else 'x64'
   return arch
 
 def to_gn_args(args):
   gn_args = {}
 
+  host_os = HOST_OS
+  if HOST_OS == 'macos':
+    host_os = 'mac'
   if args.os == 'host':
-    gn_args['target_os'] = HOST_OS
+    gn_args['target_os'] = host_os
   else:
     gn_args['target_os'] = args.os
 
   gn_args['dart_target_arch'] = args.arch
   gn_args['target_cpu'] = target_cpu_for_arch(args.arch, args.os)
   gn_args['host_cpu'] = host_cpu_for_arch(args.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.
   gn_args['dart_host_pub_exe'] = os.path.join(
-      DART_ROOT, 'tools', 'sdks', HOST_OS, 'dart-sdk', 'bin', 'pub')
+      DART_ROOT, 'tools', 'sdks', host_os, 'dart-sdk', 'bin', 'pub')
 
   # 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"
 
   gn_args['dart_use_tcmalloc'] = gn_args['target_os'] == 'linux'
 
   gn_args['is_debug'] = args.mode == 'debug'
@@ skipping 77 matching lines @@
   ]
   gn_args = to_command_line(to_gn_args(args))
   out_dir = get_out_dir(args)
   print "gn gen --check in %s" % out_dir
   command.append(out_dir)
   command.append('--args=%s' % ' '.join(gn_args))
   return subprocess.call(command, cwd=DART_ROOT)
 
 if __name__ == '__main__':
     sys.exit(main(sys.argv))