whereType<S>  method 
This transformer is a shorthand for Stream.where followed by Stream.cast.
Events that do not match T are filtered out, the resulting Stream will
be of Type T.
Example
Stream.fromIterable([1, 'two', 3, 'four'])
  .whereType<int>()
  .listen(print); // prints 1, 3
as opposed to:
Stream.fromIterable([1, 'two', 3, 'four'])
  .where((event) => event is int)
  .cast<int>()
  .listen(print); // prints 1, 3
Implementation
Stream<S> whereType<S>() => WhereTypeStreamTransformer<T, S>().bind(this);