Introduce dart_host_toolchain arg, remove '--checked' option from patch_sdk.py

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
@@ skipping 30 matching lines @@
   if utils.CheckedInSdkCheckExecutable():
     options.dart_executable = utils.CheckedInSdkExecutable()
   elif options.dart_executable is not None:
     if not options.quiet:
       DisplayBootstrapWarning()
     options.dart_executable = os.path.abspath(options.dart_executable)
   else:
     print >> sys.stderr, 'ERROR: cannot locate dart executable'
     return -1
   dart_file = os.path.join(os.path.dirname(__file__), 'patch_sdk.dart')
-  subprocess.check_call(
-      [options.dart_executable, '--checked', dart_file] + args)
+  subprocess.check_call([options.dart_executable, dart_file] + args)
   return 0
 
 if __name__ == '__main__':
   sys.exit(main())