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In Drupal 8 Entity validation is moved to a separate Entity validation API and so decoupled from form validation. Having entity validation decoupled from forms allows validation entities independently from form submissions, e.g. when changed via RESTful web service. This new validation API has been implemented based upon Symfony validator.
[WIP] - the new validation API is in place and fully functional, but it is not used for validating entity forms yet. Check for further conversion details.
Read the validation API documentation provided here.