Adds a Descriptor to the pool, non-recursively.

    If the Descriptor contains nested messages or enums, the caller must
    explicitly register them. This method also registers the FileDescriptor
    associated with the message.

    Args:
      desc: A Descriptor.

def AddDescriptor(self, desc):
    """Adds a Descriptor to the pool, non-recursively.

    If the Descriptor contains nested messages or enums, the caller must
    explicitly register them. This method also registers the FileDescriptor
    associated with the message.

    Args:
      desc: A Descriptor.
    """
    if not isinstance(desc, descriptor.Descriptor):
      raise TypeError('Expected instance of descriptor.Descriptor.')

    self._descriptors[desc.full_name] = desc
    self._AddFileDescriptor(desc.file)