MediaStreamEvent

Why is MediaStreamEvent.stream nullable? 
http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastreamevent

Unless there's a non-circular reason, I suggest we make the readonly arg 
non-nullable AND either:

 1. make the stream a required constructor arg or
 2. add prose that constructor must throw on null or absence of the
    stream init-dictionary member, or
 3. add no prose and leave it to be inferred


Then perhaps use the same pattern for the other events as well.

Minor naming side-note: I expected the newly proposed addTrack() api and 
onaddtrack to take an event named MediaStreamTrackEvent rather than 
AddTrackEvent, for consistency. Thoughts on naming there?

.: Jan-Ivar :.