migrate to null safety (dart-lang/coverage#332)

I did not do large refactors for the most part here I left in existing assumptions which means a lot of `!` are added.

Note that the `_shouldIgnoreLine` private method was moved to a local method and the patterns around it changed a fair bit due to iterators changing with null safety (it previously relied on `current` being null and now uses the result of `moveNext`).
17 files changed
tree: 461b4111c4d7f50348d524b1ec791b114c4f68ba
  1. pkgs/