Hi,
There is Google Analytics webpart present in the 5.5 R2 installation.
The info at
http://devnet.kentico.com/Marketplace/Google/Google-analytics-Web-Part.aspx really mentions this, however it seems the information is not correct. One comment also complains about it - the version in 5.5R2 is not the same as the one at Marketplace.
The asynchronous script support is only in the Marketplace webpart, it is not in the webpart included with 5.5 R2.
I have forwarded this info to responsible people so this should be corrected soon.
Anyway, it should be possible to import the Marketplace webpart into the installation to have the async.script support.
Regards,
Zdenek