$ sudo apt-get update
$ sudo apt-get install flashplugin-downloader 

flashplugin-downloader is installed in your system.

Make ensure the flashplugin-downloader package were installed using the commands given below,

$ sudo dpkg-query -l | grep flashplugin-downloader *

You will get with flashplugin-downloader package name, version, architecture and description in a table

flashplugin-downloader package basic information: