Merge branch '6.1' into 6.2

* 6.1:
  Update ComposerPlugin.php
  [Notifier] [OvhCloud] handle invalid receiver
  [Cache] fix collecting cache stats when nesting computations
  [VarDumper] Fix JS to expand / collapse
  [Validator] Fix Email validator logic
  Fix user_identifier support after username has been deprecated in favor of it.
  [Tests] Remove `$this` occurrences in future static data providers
  [PropertyInfo] Fixes constructor extractor for mixed type
  use method_exists() instead of catching reflection exceptions

Author: nicolas-grekas

Dumper/HtmlDumper.php

@@ -158,9 +158,9 @@ protected function getDumpHeader()
     };
 
 refStyle.innerHTML = 'pre.sf-dump .sf-dump-compact, .sf-dump-str-collapse .sf-dump-str-collapse, .sf-dump-str-expand .sf-dump-str-expand { display: none; }';
-(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
+doc.head.appendChild(refStyle);
 refStyle = doc.createElement('style');
-(doc.documentElement.firstElementChild || doc.documentElement.children[0]).appendChild(refStyle);
+doc.head.appendChild(refStyle);
 
 if (!doc.addEventListener) {
     addEventListener = function (element, eventName, callback) {