Weird assignment of definition['settings']['target_type'] in FieldItem::getPropertyDefinitions()

FieldItem classes extending EntityReferenceItem have code like:

FileItem:

public function getPropertyDefinitions() {
  $this->definition['settings']['target_type'] = 'file';

TaxonomyTermReferenceItem:

public function getPropertyDefinitions() {
  $this->definition['settings']['target_type'] = 'taxonomy_term';

This looks really weird. getPropertyDefinitions() is not the place to put side effects and assign missing properties in $this->definition['settings'] - what if the the Item object is used without getPropertyDefinitions() being called ?

Not sure where/how this 'target_type' property can be set, but we should look for another way.