Fix SF bug [ #450245 ] Error in parsing future stmts
Fix SF bug [ #450245 ] Error in parsing future stmts

Check return value from future_parse() in for loop for file_input to
accomodate multiple future statements on separate lines.

Add several comments explaining how the code works.

Remove out-dated XXX comment.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator