If something is listed in `__future__.all_feature_names`, and it's not in `__future__`, I'd consider that a bug in IronPython's `__future__` module. For that matter, [the docs](http://docs.python.org/2/library/__future__) say that these features have a `compiler_flag` attribute.

Perhaps it's simpler to put this in a try/except and fallback to 0.