modifyBaseField example

    $this->removeBaseFieldIndex();
    $this->assertTrue($this->entityDefinitionUpdateManager->needsUpdates(), 'EntityDefinitionUpdateManager reports that updates are needed.');
    $changes = $this->entityDefinitionUpdateManager->getChangeSummary();
    $this->assertCount(1, $changes['entity_test_update']);
    $this->assertEquals('The A new base field field needs to be updated.', strip_tags($changes['entity_test_update'][0]));
    $this->applyEntityUpdates();
    $this->assertFalse($this->database->schema()->indexExists('entity_test_update', 'entity_test_update_field__new_base_field'), 'Index deleted.');

    // Update the type of the base field from 'string' to 'text', ensure the     // update manager reports it, and the update adjusts the schema     // accordingly.     $this->modifyBaseField();
    $this->assertTrue($this->entityDefinitionUpdateManager->needsUpdates(), 'EntityDefinitionUpdateManager reports that updates are needed.');
    $changes = $this->entityDefinitionUpdateManager->getChangeSummary();
    $this->assertCount(1, $changes['entity_test_update']);
    $this->assertEquals('The A new base field field needs to be updated.', strip_tags($changes['entity_test_update'][0]));
    $this->applyEntityUpdates();
    $this->assertFalse($this->database->schema()->fieldExists('entity_test_update', 'new_base_field'), 'Original column deleted in shared table for new_base_field.');
    $this->assertTrue($this->database->schema()->fieldExists('entity_test_update', 'new_base_field__value'), 'Value column created in shared table for new_base_field.');
    $this->assertTrue($this->database->schema()->fieldExists('entity_test_update', 'new_base_field__format'), 'Format column created in shared table for new_base_field.');

    // Remove the base field, ensure the update manager reports it, and the     // update deletes the schema.