I am on Yosemite 10.10.1. For some reason (maybe corrupted initial download) the 1.2MB installer stops at around 30% (34%, 35% or 36%) downloading the full installer.
I spent some time why but I just gave up. Here is my workaround and I hope it will help others. The workaround is to download the offline installer from this page http://www.adobe.com/products/flashplayer/distribution3.html. Download the "DMG Installer (for System Administrators)" and install. After installing restart Safari for example and verify from http://www.adobe.com/uk/software/flash/about/ that you have installed.