Without the change `resolvedUrl` fails on non-mirror:// packages as:
$ nix eval --impure --expr 'with import ./. {}; AMB-plugins.src.resolvedUrl'
error:
...
error: expected a list but found null: null
After ther change the eval works as expected:
$ nix eval --impure --expr 'with import ./. {}; AMB-plugins.src.resolvedUrl'
"http://kokkinizita.linuxaudio.org/linuxaudio/downloads/AMB-plugins-0.8.1.tar.bz2"