Strict warnings

I improved the daily build/test for the search_api family. While the tests for search_api_solr and search_api_solr_multilingual are passing, the search_api and facets tests partly fail:
https://travis-ci.org/mkalkbrenner/search_api_integration_tests/builds/1...

An interesting fact are a lot of Exceptions caused by these three strict warnings:

    Exception Strict war IndexAddFieldsFor  433 include()                          
    Drupal\Core\Entity\EntityForm and
    Drupal\search_api\Form\UnsavedConfigurationFormTrait define the same
    property ($entityTypeManager) in the composition of
    Drupal\search_api\Form\IndexAddFieldsForm. This might be incompatible, to
    improve maintainability consider using accessor methods in traits instead.
    Class was composed

    Exception Strict war IndexFieldsForm.p  435 include()                          
    Drupal\Core\Entity\EntityForm and
    Drupal\search_api\Form\UnsavedConfigurationFormTrait define the same
    property ($entityTypeManager) in the composition of
    Drupal\search_api\Form\IndexFieldsForm. This might be incompatible, to
    improve maintainability consider using accessor methods in traits instead.
    Class was composed

    Exception Strict war FieldConfiguratio  173 include()                          
    Drupal\Core\Entity\EntityForm and
    Drupal\search_api\Form\UnsavedConfigurationFormTrait define the same
    property ($entityTypeManager) in the composition of
    Drupal\search_api\Form\FieldConfigurationForm. This might be incompatible,
    to improve maintainability consider using accessor methods in traits
    instead. Class was composed