Summary: Shows events which a page dispatched, plus all the registered
listeners.
Test Plan:
Pretty basic for now, but works OK:
https://secure.phabricator.com/file/view/PHID-FILE-49fcd23081ce55cf9369/
(I also made it dispatch some dummy events to verify they show up.)
Reviewers: aran
Reviewed By: aran
CC: aran
Differential Revision: 973
