field_get_default_value() is replaced by FieldDefinitionInterface::getDefaultValue

In Drupal 7, field_get_default_value() could be used to retrieve the default value that was automatically applied to a field in a newly created entity.
In Drupal 8, this can be accessed with the getDefaultValue() method on FieldDefinitionInterface objects.

Note that the Entity API takes care of automatically applying default values on newly created entities, so there should be few use cases for fetching this information directly.

Drupal 7

$default = field_get_default_value($entity, $field, $instance, $langcode);

Drupal 8

$default = $field_definition->getDefaultValue($entity);

See also: https://drupal.org/node/2031221