Creates a new stream from this stream that discards some data events.
The new stream sends the same error and done events as this stream,
but it only sends the data events that satisfy the test.
The returned stream is a broadcast stream if this stream is.
If a broadcast stream is listened to more than once, each subscription
will individually perform the test.
Source
Stream<T> where(bool test(T event)) {
return new _WhereStream<T>(this, test);
}