Having problem with namespace
The answer is in the error message: 

FieldItemBase has a constructor, and it waits a parameter (actually multiple), but it doesn't get any: 

$instance = new FileItem();


Even though the constructor function can't be find in FileItem, in this case you have to investigate which parent has it. You will find out that "FieldItemBase" has it. 
I don't know why you want to use this like this, but things like Controllers, Plugins, Services were mean to be to get instantiated by Drupal.