Drop the _WhereType transformer definition (dart-lang/stream_transform#98)

This was only required because of the challenges of being exposed as a
`StreamTransformer` and the subtyping rules for the generic arguments.
Now that we aren't exposing it as `StreamTransformer` we don't need
extra complications.
2 files changed
tree: cc9264e80ca85d1ca4054db8967d810f7218ff08
  1. pkgs/