Run GN from build.py if the output directory doesn't exist

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 375 matching lines @@
       "$n.showballoontip(%d, '%s', '%s', "
       "[system.windows.forms.tooltipicon]::%s);\"") % (
         5000, # Notification stays on for this many milliseconds
         message, title, icon)
 
   if command:
     # Ignore return code, if this command fails, it doesn't matter.
     os.system(command)
 
 
+def RunGN(target_os, mode, arch):
+  gn_command = ['tools/gn.py',
+    '-m', mode,
+    '-a', arch,
+    '--os', target_os,
+    '-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)
+  if not os.path.exists(out_dir):
+    RunGN(target_os, mode, arch)
   command = ['ninja', '-C', out_dir]
   if options.verbose:
     command += ['-v']
   command += [target]
   return command
 
 
 filter_xcodebuild_output = False
 def BuildOneConfig(options, target, target_os, mode, arch, override_tools):
   global filter_xcodebuild_output
@@ skipping 56 matching lines @@
               options.j,
               'BUILDTYPE=' + build_config,
               ]
       if target_os != HOST_OS:
         args += ['builddir_name=' + utils.GetBuildDir(HOST_OS)]
       if options.verbose:
         args += ['V=1']
 
       args += [target]
 
-  toolsOverride = None
-  if override_tools:
-    toolsOverride = SetTools(arch, target_os, options)
-  if toolsOverride:
-    for k, v in toolsOverride.iteritems():
-      args.append(  k + "=" + v)
-      if options.verbose:
-        print k + " = " + v
-    if not os.path.isfile(toolsOverride['CC.target']):
-      if arch == 'arm':
-        print arm_cc_error
-      else:
-        print "Couldn't find compiler: %s" % toolsOverride['CC.target']
-      return 1
-
+    toolsOverride = None
+    if override_tools:
+      toolsOverride = SetTools(arch, target_os, options)
+    if toolsOverride:
+      for k, v in toolsOverride.iteritems():
+        args.append(  k + "=" + v)
+        if options.verbose:
+          print k + " = " + v
+      if not os.path.isfile(toolsOverride['CC.target']):
+        if arch == 'arm':
+          print arm_cc_error
+        else:
+          print "Couldn't find compiler: %s" % toolsOverride['CC.target']
+        return 1
 
   print ' '.join(args)
   process = None
   if filter_xcodebuild_output:
     process = subprocess.Popen(args,
                                stdin=None,
                                bufsize=1, # Line buffered.
                                stdout=subprocess.PIPE,
                                stderr=subprocess.STDOUT)
     FilterEmptyXcodebuildSections(process)
@@ skipping 73 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())