Allow null values in asyncMapSample and buffer (dart-lang/stream_transform#124)

Closes dart-lang/stream_transform#121

- Add a boolean to track whether we have a value since we can't use
  `null` as a sentinel value for a potentially nullable generic type.
- Replace `!` with `as S` to allow for nullable `S`.
4 files changed
tree: eff2079c695988d20669673993d05ad1ec1e8211
  1. pkgs/