I have download ADT 21.1, and tried install it on eclipse
I got those:
I also confirm that this works (already mentioned by ing0)
In Eclipse (ADT)
Help --> Install New Software ---> Work with --> Android Developer Tools Update Site - http://dl-ssl.google.com/android/eclipse/
Select all of the updates you want to install and UNCHECK option "Contact all update sites during install to find required software". After that everything should install as it is mentioned to be.