Hi Lena,
the explanation is simple. The link you provided is based on the
discontinued "Google code search" service (see
http://www.google.com/codesearch). Apparently this service is no longer
available but some of the old links are still working. In your case the
link shows an outdated view of our SVN repository as it looked like
months ago.
Meanwhile the TileDownloader feature has been removed from the
development trunk (please read the mapsforge-dev archive if you are
interested in the rationale behind this decision).
To cut a long story short: in order to see the current contents of our
SVN repository, just go to the "Source" tab on our Google project page:
https://code.google.com/p/mapsforge/source/browse/
Greetings,
Thilo