Null safe API (dart-lang/markdown#347)

Migrate API to null safety.

This does not include some files in bin/, test/, and tool/:

* bin/markdown.dart requires package:args, which has not published a null safe release.
* tool/stats.dart requires package:args.
* tool/stats_lib.dart requires package:html, which has not published a null safe release.
* tool/dartdoc-compare.dart requires package:args.
* test/util.dart requires package:io, which has not published a null safe release.
* test/markdown_test.dart, test/vesion.dart import test/util.dart.

Most APIs upgraded to null safety very smoothly. Very clean change.