The following code runs perfectly in Concrete5 since version 5.6.x:

Loader::model('stacks');
$globalstack = Stack::getOrCreateGlobalArea('NameOfGlobalArea');

Now returns two errors:
( ! ) Warning: require_once(C:\wamp\www\mysite\concrete/models/stacks.php): failed to open stream: No such file or directory in C:\wamp\mysite\theme\concrete\core\libraries\loader.php on line 40

( ! ) Fatal error: require_once(): Failed opening required 'C:\wamp\www\mysite\concrete/models/stacks.php' (include_path='C:/wamp/www/mysite/libraries/3rdparty;C:\wamp\www\mysite\concrete/libraries/3rdparty;.;C:\php\pear') in C:\wamp\www\mysite\concrete\core\libraries\loader.php on line 40

I also see that concrete/models/stack now not exists.

Thanks!