Adds a capturing subscription to this stream.
If the target of the event is a descendant of the element from which this
stream derives then
onData is called before the event propagates down to
the target. This is the opposite of bubbling behavior, where the event
is first processed for the event target and then bubbles upward.
- Event Capture from the W3C DOM Events specification.
StreamSubscription<T> capture(void onData(T event));