Change record status: 
Introduced in branch: 

Field constant for cardinality was located in field.module. To clean up and get it under the responsible class, it was renamed and moved to Drupal\Core\Field\FieldStorageDefinitionInterface.

Drupal 7 and 8 before the change:

if ($element['#cardinality'] != FIELD_CARDINALITY_UNLIMITED) {

Drupal 8 after:

use Drupal\Core\Field\FieldStorageDefinitionInterface;

if ($element['#cardinality'] != FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED) {

Moreover a new method isMultiple() has been added to Drupal\Core\Field\FieldStorageDefinitionInterface in order to clean up the code.

Before the change:

if ($field->getFieldCardinality() > 1 || $field->getFieldCardinality() == FIELD_CARDINALITY_UNLIMITED) {

After the change:

if ($field_storage->isMultiple()) {
Module developers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other updates done