@IvanGoncharov any special reason for this decision?

@jgcmarins It will help to prevent issues like #2522 and make this repo friendlier for contributors who never used Yarn.
Also, my personal philosophy says: "The fewer dependencies the better" and since we can't drop npm it means we should drop yarn.
Finally, since npm was bought by GitHub they intensify their development cycle and published a very promising roadmap.

Thanks