CKEditor not displaying in latest 8.0.x-dev version
Here is the correct code to create a WYSIWYG field:

$fields['title'] = BaseFieldDefinition::create('text_long')
  -&gt;setLabel(t('Content'))
  -&gt;setDescription(t('The content of the block'))
  -&gt;setDisplayOptions('form', [
      'type' =&gt; 'text_textarea',
    ])
  -&gt;setDisplayOptions('view', [
      'type' =&gt; 'text_default',
    ])
  -&gt;setDisplayConfigurable('form', TRUE)
  -&gt;setDisplayConfigurable('view', TRUE);