Allow usage of entity_reference field widget and formatter for base fields

Hit this as well today and came up with the same fix. If this is green, RTBC.

Awesome work, @andypost!!!
I personally think that per #13 this is in fact a bug.

Yeah, the dependency on entity_reference is unfortunate and yes, using the widget would require to change all those to username (id).

It just seems very weird to add faked field definitions to the EntityTestBaseFieldDisplay class.

Maybe as a middle thing, we can alter user_id and add the widget there and test with that in that test?

1. +++ b/core/modules/entity_reference/lib/Drupal/entity_reference/Tests/EntityReferenceAutocompleteTest.php
   @@ -130,6 +132,30 @@ protected function getAutocompleteResult($type, $input) {
   +    //entity_test_create_bundle('test_bundle');
   

   left-over?

2. +++ b/core/modules/entity_reference/lib/Drupal/entity_reference/Tests/EntityReferenceAutocompleteTest.php
   @@ -130,6 +132,30 @@ protected function getAutocompleteResult($type, $input) {
   +
   +    $request = Request::create('entity_reference/autocomplete/single/user_id/entity_test/entity_test/NULL');
   +    $request->query->set('q', 'auto');
   +
   +    $entity_reference_controller = EntityReferenceController::create($this->container);
   +    $result = $entity_reference_controller->handleAutocomplete($request, 'single', 'user_id', 'entity_test', 'entity_test', 'NULL')->getContent();
   +
   

   While we're adding quasi-unit test coverage for this, can we also test some of the error handling in there? like trying to pass in a non-existing field and verifiy it throws an exception?

This looks great. But parts of it are based on my ideas/suggestions, a 1+ from someone else would be nice before I RTBC it.

Looks great to me!

One super minor nitpick, but it doesn't hurt anyone so RTBC.

+++ b/core/modules/entity_reference/tests/modules/entity_reference_test/entity_reference_test.module
@@ -4,3 +4,21 @@
+      ->setDisplayConfigurable('form', TRUE);

AFAIK this is not even necessary, setDisplayOptions() should suffice, I think.

This looks like a good clean-up. I asked berdir and there's not really a way to manually test this in the UI, but the test coverage looks good.

Committed and pushed to 8.x. Thanks!