nuget.exe install Microsoft.Office.js
As of this writing (updated on May 8th, 2017) this created a folder in the C:\Apps folder called Microsoft.Office.js.188.8.131.52. That was it. Simple. I then pulled over those files as a zip to the customer project a installed them.
NOTE: The NuGet version of the Office.js libraries are not necessarily the newest. The latest are always available on the CDN, however there is not a way to pull those copies down for a local version. The NuGet version is updated with each point release, but it will lag at some interval behind what is available on CDN.