Cleanup based on breaking changes

Hello @financeCoding and @kevmoo, Could you have a look at my pull request?

I attempted some cleanup based on breaking change announcements:

[BREAKING CHANGE: dart:io LineTransformer is deprecated, replaced by dart:convert LineSplitter](https://groups.google.com/a/dartlang.org/forum/#!msg/misc/InyyFhhThGE/On8LTrb_iRAJ)
[BREAKING CHANGE: dart:io StringEncoder / StringDecoder removed](https://groups.google.com/a/dartlang.org/forum/#!searchin/misc/StringDecoder/misc/X8fwtInnl7I/94j_rRvOmmQJ)