RS, RT> method Null safety
StreamTransformer<RS, RT> view of this stream transformer.
The resulting transformer will check at run-time that all data events
of the stream it transforms are actually instances of
and it will check that all data events produced by this transformer
are actually instances of
StreamTransformer<RS, RT> cast<RS, RT>() => StreamTransformer.castFrom<S, T, RS, RT>(this);