Creates a new stream that converts each element of this stream
to a new value using the convert function.
For each data event, o, in this stream, the returned stream
provides a data event with the value convert(o).
If convert throws, the returned stream reports the exception as an error
event instead.
Error and done events are passed through unchanged to the returned stream.
The returned stream is a broadcast stream if this stream is.
The convert function is called once per data event per listener.
If a broadcast stream is listened to more than once, each subscription
will individually call convert on each data event.
Source
Stream<S> map<S>(S convert(T event)) {
return new _MapStream<T, S>(this, convert);
}