Make it visible whether a stream is controlled by a synchronous controller.

[lrhn]

If we have stream controllers that promise to deliver events immediately, make it possible to detect this on the stream.

Some functionality has been written that is only intended for streams controllers where the input and output events are never separated in time. There is no way to recognize this in the current classes.

This requires a conceptual change because the stream itself is currently not intended for delivering events that are distinguishable from being delivered in a later microtask. The mentioned functionality is really not working on a "Stream" but on another concept with the same API.