D8: Get the current user ID in theme file and set a variable via MYTHEME_preprocess_page to use it in page.html.twig.

page.html.twig

{# isAdmin? #}
{% if UserID == '1' %}
  {{ kint() }}
{% endif %}

theme.php

/**
 * Implements hook_preprocess_HOOK() for page templates.
 */
function MYTHEME_preprocess_page(&$variables) {
    $UserID = $variables['user']->id();
    $variables['UserID'] = $UserID;
}