Making sure temporary named file is created with 'w' mode.
Currently, this causes permission error on Windows platform
Merge pull request #621 from NghiaNguyen/master

Making sure temporary named file is created with 'w' mode.
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
addressed comments
added another test case
Incorporated suggestions in parsing rules