Get instance of builder class based on flags.

def GetBuilder(self, context, fleetspeak_service_config):
    """Get instance of builder class based on flags."""
    try:
      if "Target:Darwin" in context:
        return builders.DarwinClientBuilder(
            context=context,
            fleetspeak_service_config=fleetspeak_service_config)
      elif "Target:Windows" in context:
        return builders.WindowsClientBuilder(context=context)
      elif "Target:LinuxDeb" in context:
        return builders.LinuxClientBuilder(context=context)
      elif "Target:LinuxRpm" in context:
        return builders.CentosClientBuilder(context=context)
      else:
        parser.error("Bad build context: %s" % context)
    except AttributeError:
      raise RuntimeError("Unable to build for platform %s when running "
                         "on current platform." % self.platform)