After #1876018: Displaced space in core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php has been committed I stumble upon lots of "disabled spaces". Both ways are possible but I think that we have to agree on a standard for that.
$ grep -irn "= &" drupal-8.x | wc -l
467$ grep -irn "=&" drupal-8.x | wc -l
68The results include some regex expresions but you can see that the = & way is more present than the other way. After the code freeze we can start with writing patches for that. Note that this part is missing in the coding standards, so we also have to update them.