Thanks for the quick turn around. Couple of things
1. The link points to a deprecated method of packaging power bi visuals. And it partly has same issues as (2) below.
2. The new tutorial is about creating a custom visual from scratch and packaging it. I have tried it, I am able to create the visual from scratch and package it. It works great. (My bad, I should had mentioned it).
But what I am really looking for is to make real minor change to existing visuals that power bi already has and package it.
As I have already mentioned, I have managed to apply a change to an existing power bi visual (change font size of tree map label) and that works as expected with the test page available in the source code. But my concern is the source code I got from github is marked deprecated. So can you please point me to the right version of source code and also how do I package it after making my change to existing power bi visual like tree map.