Support symfony/YAML 3 in BlockContentCreator (Drupal Core 8.4)

When using Symfony/Yaml::parse the input needs to be a string containing all the YAML. This means that just inserting the filepath is not working anymore.

Solution:

Change every Yaml::parse call from:

<?php
$source = "{$this->path}/{$this->id}.yml";
$this->data = Yaml::parse($source);
?>

To:

<?php
$source = "{$this->path}/{$this->id}.yml";
$content = file_get_contents($source);
$this->data = Yaml::parse($content);
?>