Why does adding a dashboard widget in includes/admin.php fail?

Question

Does includes/admin.php have special status I don't understand?

This is my dash widget code:

add_action('wp_dashboard_setup', 'custom_dashboard_widgets');

function custom_dashboard_widgets() {
  global $wp_meta_boxes;
  error_log("I love testing!");
  wp_add_dashboard_widget('test_dash', 'Test Dash', 'test_dash');
}

function test_dash(){
  echo "Test";
}
?>

If I put that code in includes/dash.php it works perfectly. If I put it in includes/admin.php nothing happens. Both files are included in the plugin using the same method:

require_once('includes/admin.php');
require_once('includes/dash.php');

Answer 1

The problem is here:

require_once('includes/admin.php');

It should be:

require_once(plugin_dir_path( __FILE__ ).'/includes/admin.php');

The original code actually sources a core WordPress file with the same name.