Change build.py to invoke gn with '--check' to match the build bot's script and Flutter's usage.

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, 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 optparse
 import os
 import re
@@ skipping 390 matching lines @@
 
 
 def RunGN(target_os, mode, arch):
   gn_os = 'host' if target_os == HOST_OS else target_os
   gn_command = [
     'python',
     os.path.join(DART_ROOT, 'tools', 'gn.py'),
     '-m', mode,
     '-a', arch,
     '--os', gn_os,
+    '--check',
     '-v',
   ]
   process = subprocess.Popen(gn_command)
   process.wait()
   if process.returncode != 0:
     print ("Tried to run GN, but it failed. Try running it manually: \n\t$ " +
            ' '.join(gn_command))
 
 def BuildNinjaCommand(options, target, target_os, mode, arch):
   out_dir = utils.GetBuildRoot(HOST_OS, mode, arch, target_os)
@@ skipping 175 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())