Remove usage of field_ui_default_value and recommend proper replacement

Well issue has fail patch and pass patch so bug is fixed but I don't know a lot about this issue leaving it for @swentel or @yched to RTBC.

in_array('default_value_input', $form['#parents']) is one way to differentiate, yes.
Alternatively, we could also use if (empty($form_state['default_value_widget']), that's what I did for LinkWidget in #2293723: Generate lighter $form[$field] structures.

I hesitated between the two back then in that issue, and considered that a condition on $form_state['default_value_widget'] was a bit more telling (and not dependent on the form tree structure). No big difference I guess, but we should be consistent.

"But we should be consistent"...

I guess one way would be to encapsulate this check once and for all in a isDefaultValueWidget() method on WidgetBase.
- widgets that need this info, like LinkWidget, can call it directly instead of doing an obscure test
- we can pass that as $context['default'] = $this->isDefaultValueWidget() when calling hook_field_widget_form_alter()

Patch needs to be updated.

Here is the updated patch.

+++ b/core/modules/field_ui/src/Tests/ManageFieldsTest.php
@@ -569,6 +569,11 @@ function testHelpDescriptions() {
+    // Check that hook_field_widget_form_alter() does believe this is the
+    // default value form.
+    $this->drupalGet('admin/structure/types/manage/article/fields/node.article.field_tags');
+    $this->assertText('From hook_field_widget_form_alter(): Default form is true.', 'Default value form in hook_field_widget_form_alter().');
+

I was a little confused what this code was testing. Instead of hook_field_widget_form_alter(), could the comment mention field_test_field_widget_form_alter()?

Also, from #10

"But we should be consistent"...

should be addressed I think.

Looks good. Thanks folks :-)

Committed/pushed to 8.0.x, thanks!

Follow-up looks good but please open a new issue. Two patches means two commit hashes.