Drop checks for unsound null values (dart-lang/stream_transform#189)

Towards dart-lang/stream_transform#111

During the null safety migration it was possible for
`StreamSubscription.cancel()` to return `null` in some cases. Now that
this package is only used with sound null safe code it isn't possible
for these nulls to flow through anymore so it is not necessary to filter
for them.
5 files changed
tree: 074ded649971999f808622c2f034a36e8c598de5
  1. pkgs/