Drop custom dart_host_sdk used for Flutter.

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 shutil
 import subprocess
@@ skipping 129 matching lines @@
   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'] = TargetCpuForArch(arch, target_os)
   gn_args['host_cpu'] = HostCpuForArch(arch)
   crossbuild = gn_args['target_cpu'] != gn_args['host_cpu']
 
-  # 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'] = ""
-
   if arch != HostCpuForArch(arch):
     # Training an app-jit snapshot under a simulator is slow. Use script
     # snapshots instead.
     gn_args['dart_snapshot_kind'] = 'script'
   else:
     gn_args['dart_snapshot_kind'] = 'app-jit'
 
   # We only want the fallback root certs in the standalone VM on
   # Linux and Windows.
   if gn_args['target_os'] in ['linux', 'win']:
@@ skipping 302 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))