Replace calls to form_set_error() to $form_state->setErrorByName()

Changes look good. I just have a small nitpick to point out:

+++ b/core/modules/views/src/Plugin/views/pager/SqlBase.php
@@ -195,16 +195,14 @@ public function validateOptionsForm(&$form, &$form_state) {
     }
...
-        form_set_error('pager_options][expose][items_per_page_options', $form_state, t('Insert the items per page (@items_per_page) from above.',
-            array('@items_per_page' => $items_per_page))
-        );
+        \Drupal::formBuilder()->setErrorByName('pager_options][expose][items_per_page_options', $form_state, t('Insert the items per page (@items_per_page) from above.', array('@items_per_page' => $items_per_page)));

Should we retain the line break for array here?

I'm in favor of doing it. AFAIK if the length of the line containing array is more than 80 chars, then the array must be split to a new line.

Agree that you version in #5 is the better way to do it.

rerolled. replaced two more "form_set_error" calls, fixed indentation, and removed a few optional parameters.

Please wait on this, we're changing how setErrorByName works in #2225353: Convert $form_state to an object and provide methods like setError()

update after #2225353: Convert $form_state to an object and provide methods like setError().

Last submitted patch worked fine.No trace of form_set_error found.

+++ b/core/modules/system/system.module
@@ -8,6 +8,7 @@
@@ -1120,16 +1121,17 @@ function system_check_directory($form_element) {

@@ -1120,16 +1121,17 @@ function system_check_directory($form_element) {
+  $form_state = new FormState();
   $logger = \Drupal::logger('file system');

Actaully this method is used in a #after_build callback and the second argument is the form state so let's fix that. This looks wrong. See views_ui_add_ajax_wrapper() for an example of an #after_build function.

fixed #12.

So can we please remove the function as well, to avoid adding it back again?

Sure.

Thanks! (RTBC if it passes)

Committed 7fff6e8 and pushed to 8.0.x. Thanks!