Move token builder functions to a service

Looks like a good start. Some specific feedback below.

AFAIK, we currently have 0 test coverage for the token tree. Would be great to start adding some UI tests there, could be a pretty simple start, e.g. asserting that a bunch of tokens and nested tokens show up on the page. Then we can extend it with regression tests if we ever have to fix tests or add new features there. Want to open a follow-up for that?

1. +++ b/src/TreeBuilder.php
   @@ -0,0 +1,179 @@
   +
   +  public function __construct(ContainerInterface $container) {
   +    $this->container = $container;
   +  }
   

   Please inject the services that we actually need.

2. +++ b/src/TreeBuilder.php
   @@ -0,0 +1,179 @@
   +  public function buildTree($token_type, array $options = []) {
   +    $trees = &$this->builtTrees;
   

   weird drupal_static() leftover.

3. +++ b/src/TreeBuilder.php
   @@ -0,0 +1,179 @@
   +      if (isset($token_info['children']) && is_array($token_info['children'])) {
   +        $result += static::flattenTree($token_info['children']);
   

   should be $this->flattenTree()

4. +++ b/token.module
   @@ -625,143 +625,20 @@ function token_form_user_admin_settings_alter(&$form, FormStateInterface $form_s
    function token_flatten_tree($tree) {
   

   pretty sure those functions are just internal helper functions for the theme functions, I don't think we need to keep them around.

Committed. Nice, one step closer to unit testable code.

I think you're already working on converting the remaining tree theme code there to twig where you will probably convert these calls. But lets also do a follow-up to convert all other calls to this function, including properly injecting the service into controllers and so on.