firebase deploy progress shows NaN/undefined sometimes

[bmeurer]

Using firebase deploy on a project sometimes gives a weird progress, see screenshot.

Version info

$ firebase --version
6.1.0
$ node --version
v10.1.0

Platform Information

macOS 10.13.6

Steps to reproduce

Doesn't happen all the time, got it like 10% of the times with just

firebase deploy

Expected behavior

Show proper progress, i.e. 123/321

Actual behavior

Shows messed up progress NaN/undefined (the NaN is probably there because of undefined is converted to a Number somewhere).

[bkendall]

Dupe of #1020