6 files declare their use of FieldInstance

field.info.inc in drupal/core/modules/field/field.info.inc
Field Info API, providing information about available fields and field types.
field_ui.admin.inc in drupal/core/modules/field_ui/field_ui.admin.inc
Administrative interface for custom field type creation.
FormatterBase.php in drupal/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterBase.php
Definition of Drupal\field\Plugin\Type\Formatter\FormatterBase.
FormatterInterface.php in drupal/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterInterface.php
Definition of Drupal\field\Plugin\Type\Formatter\FormatterInterface.
WidgetBase.php in drupal/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetBase.php
Definition of Drupal\field\Plugin\Type\Widget\WidgetBase.
WidgetInterface.php in drupal/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetInterface.php
Definition of Drupal\field\Plugin\Type\Widget\WidgetInterface.