This feature added complexity to the framework but wasn't used in the core anyway.
You can still use the Map class loader in your application though. But most of the time, using the APC
autoloader is just better.
A class in Symfony2 can be loaded by four different mechanisms:
* bootstrap.php: This file contains classes that are always required and
needed very early in the request handling;
* classes.php: This file contains classes that are always required and
managed by extensions via addClassesToCompile();
* MapFileClassLoader: This autoloader uses a map of class/file to load
classes (classes are managed by extensions via addClassesToAutoloadMap(),
and should contain often used classes);
* UniversalAutolaoder: This autoloader loads all other classes (it's the
slowest one).
