Added basic UI for Nestable Async Events

This CL adds basic UI support for nestable async events.
Still need to find out whether the UI looks okay. But right
now it looks like the picture in the design doc below.

Design Doc:
https://docs.google.com/document/d/1Z2uqj59UEts5IiXX78mkdU4kd6e7kE3JUKPoDK97bVs/edit?usp=sharing
Hmm, I don't think we want to convert the event phase here. We should log a console error and handle appropriately. (What does the other begin/end code do with the event when there is no start to the pair?)