mapNotNull<R extends Object>  method 
Returns a Stream containing only the non-null results
of applying the given transform function to each element of this Stream.
Example
Stream.fromIterable(['1', 'two', '3', 'four'])
  .mapNotNull(int.tryParse)
  .listen(print); // prints 1, 3
// equivalent to:
Stream.fromIterable(['1', 'two', '3', 'four'])
  .map(int.tryParse)
  .whereType<int>()
  .listen(print); // prints 1, 3
Implementation
Stream<R> mapNotNull<R extends Object>(R? Function(T) transform) =>
    MapNotNullStreamTransformer<T, R>(transform).bind(this);