public function __toString()
{
$e = new Element('link');
$e->rel('stylesheet')->type('text/css')->href($this->getAssetURL());
if (count($this->combinedAssetSourceFiles)) {
$source = '';
foreach ($this->combinedAssetSourceFiles as $file) {
$source .= $file . ' ';
}
$source = trim($source);
$e->setAttribute('data-source', $source);
}
return (string) $e;
}