@classmethod<EOL><INDENT>def _unknown_property(cls, name):<DEDENT>
raise InvalidPropertyError('<STR_LIT>' % name)<EOL>
Internal helper to raise an exception for an unknown property name.

        This is called by _check_properties().  It is overridden by
        Expando, where this is a no-op.

        Raises:
          InvalidPropertyError.