remove uses of each() function

Thank you for trying to proactive with future changes of PHP!

1. +++ b/core/lib/Drupal/Component/Diff/Engine/DiffEngine.php
   @@ -201,30 +201,31 @@ protected function _diag($xoff, $xlim, $yoff, $ylim, $nchunks) {
   +        foreach ($matches as $y) {
   +          if (!$found_empty && empty($this->in_seq[$y])) {
   ...
   -            break;
   +            $found_empty = TRUE;
   

   It would be nice if you could try to minimize the amount of changes. As far as I know break; also works for foreach. You could reduce the diff here a bit, by really just replacing while with a foreach, see https://3v4l.org/b7EkO. list() is totally usable in a foreach loop as well.

2. +++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
   @@ -94,7 +94,7 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
   -      while (list($module) = each($module_list)) {
   +      foreach ($module_list as $module => $_) {
   
   @@ -331,7 +331,7 @@ public function uninstall(array $module_list, $uninstall_dependents = TRUE) {
   -      while (list($module) = each($module_list)) {
   +      foreach ($module_list as $module => $_) {
   
   +++ b/core/lib/Drupal/Core/Extension/ThemeInstaller.php
   @@ -121,7 +121,7 @@ public function install(array $theme_list, $install_dependencies = TRUE) {
   -      while (list($theme) = each($theme_list)) {
   +      foreach ($theme_list as $theme => $_) {
   

   What about using array_keys() here?

yeah let's mark this one as duplicate of the other.