Issue #2137309 by fago: Typed data does not handle set() and onChange() consistently

core/lib/Drupal/Core/Config/Schema/Mapping.php

core/lib/Drupal/Core/Entity/ContentEntityBase.php

.../lib/Drupal/Core/Entity/Plugin/DataType/EntityAdapter.php

core/lib/Drupal/Core/Field/FieldItemBase.php

...Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php

...Drupal/Core/Field/Plugin/Field/FieldType/LanguageItem.php

core/lib/Drupal/Core/TypedData/ComplexDataInterface.php

core/lib/Drupal/Core/TypedData/Plugin/DataType/Map.php

...ules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php

core/modules/system/src/Tests/Entity/EntityFieldTest.php

.../modules/text/src/Plugin/Field/FieldType/TextItemBase.php