EntityReferenceItem::calculateDependencies() assumes $field_definition is FieldConfigBase

Problem/Motivation

FieldItemInterface::calculateDependencies() has a parameter FieldDefinitionInterface $field_definition. In EntityReferenceItem, the implementation uses $field_definition->default_value, which only exists on FieldConfigBase. When calling the method with e.g. a BaseFieldDefinition object as parameter, a PHP notice is thrown. I want to do this in #2478507: Schema configs missing dependencies to property and processor providers (collect) where configured field definitions may have been normalized and then denormalized to BaseFieldDefinition objects.

I was going to suggest to use the method getDefaultValue(), but it takes a mandatory $entity parameter. ERItem is bypassing the API here in a way, and for a reason, that are not yet clear to me. It is introduced here: #2271419-34: Allow field types, widgets, formatters to specify config dependencies

Proposed resolution

Add an isset() before using default_value.
Maybe document the API deviation.

Remaining tasks

User interface changes

API changes