Replace usages of \Drupal::menuTree() with IoC injection

Assigning to myself for review.

Thanks for the patch.

1) Changes looks fine.

2) Patch applies cleanly to 8.9, 9.0, and 9.1.

[mac:kristen:drupal-8.9.x-dev]$ patch -p1 < 3123211-2.patch 
patching file core/modules/menu_ui/src/MenuForm.php

[mac:kristen:drupal-9.0.x-dev]$ patch -p1 < 3123211-2.patch 
patching file core/modules/menu_ui/src/MenuForm.php

[mac:kristen:drupal-9.1.x-dev]$ patch -p1 < 3123211-2.patch 
patching file core/modules/menu_ui/src/MenuForm.php

3) Searching for \Drupal::menuTree():

./core/tests/Drupal/Tests/Core/DrupalTest.php:    $this->assertNotNull(\Drupal::menuTree());
./core/tests/Drupal/KernelTests/Core/Menu/MenuLinkDefaultIntegrationTest.php:    $tree = \Drupal::menuTree()->load('test', new MenuTreeParameters());
./core/tests/Drupal/KernelTests/Core/Menu/MenuLinkDefaultIntegrationTest.php:    $tree = \Drupal::menuTree()->load('test', new MenuTreeParameters());
./core/lib/Drupal/Core/Menu/menu.api.php: * $menu_tree = \Drupal::menuTree();
./core/modules/views_ui/tests/src/Functional/DisplayPathTest.php:    $result = \Drupal::menuTree()->load('admin', $parameters);
./core/modules/menu_link_content/tests/src/Kernel/MenuLinkContentDeriverTest.php:    $menu_tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/MenuLinkContentDeriverTest.php:    $menu_tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/MenuLinkContentDeriverTest.php:    $menu_tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/MenuLinkContentDeriverTest.php:    $menu_tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/MenuLinksTest.php:    $this->assertCount(1, \Drupal::menuTree()->load('menu_test', $menu_tree_condition));
./core/modules/menu_link_content/tests/src/Kernel/MenuLinksTest.php:    $this->assertCount(1, \Drupal::menuTree()->load('menu_test', $menu_tree_condition_collection));
./core/modules/menu_link_content/tests/src/Kernel/MenuLinksTest.php:    $this->assertCount(0, \Drupal::menuTree()->load('menu_test', $menu_tree_condition));
./core/modules/menu_link_content/tests/src/Kernel/MenuLinksTest.php:    $this->assertCount(1, \Drupal::menuTree()->load('menu_test', $menu_tree_condition_collection));
./core/modules/menu_link_content/tests/src/Kernel/MenuLinksTest.php:    $menu_tree = \Drupal::menuTree()->load('menu_test', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/MenuLinkContentCacheabilityBubblingTest.php:    $menu_tree = \Drupal::menuTree();
./core/modules/menu_link_content/tests/src/Kernel/PathAliasMenuLinkContentTest.php:    $tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/PathAliasMenuLinkContentTest.php:    $tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/menu_link_content/tests/src/Kernel/PathAliasMenuLinkContentTest.php:    $tree = \Drupal::menuTree()->load('tools', new MenuTreeParameters());
./core/modules/system/tests/src/Functional/System/AdminTest.php:    $menu_tree = \Drupal::menuTree();
./core/modules/system/system.module:  $menu_tree = \Drupal::menuTree();
./core/modules/views/tests/src/Kernel/Plugin/DisplayPageTest.php:    $tree = \Drupal::menuTree()->load('admin', new MenuTreeParameters());

I went through all of these to check if any could be changed to dependency injection but didn't see any.

4) Tests pass.

5) Marking RTBC.

1. I confirmed that the menu tree service is already available in MenuForm and that we're calling it correctly:

   public function __construct(MenuLinkManagerInterface $menu_link_manager, MenuLinkTreeInterface $menu_tree, LinkGeneratorInterface $link_generator, MenuLinkContentStorageInterface $menu_link_content_storage) {
     $this->menuLinkManager = $menu_link_manager;
     $this->menuTree = $menu_tree;
     $this->linkGenerator = $link_generator;
     $this->menuLinkContentStorage = $menu_link_content_storage;
   }
   

2. I also confirmed that we still prefer \Drupal::whateverService() in tests to injecting the services, because of container rebuilds etc. So that means:

   [ibnsina:drupal | Sat 13:04:08] $ grep -r "Drupal::menuTree" * | grep -v "tests"
   core/lib/Drupal/Core/Menu/menu.api.php: * $menu_tree = \Drupal::menuTree();
   core/modules/system/system.module:  $menu_tree = \Drupal::menuTree();

3. Since we don't have to inject the service anywhere new, this is a transparent change and can be backported during beta. (If we'd had to inject the service anywhere new, it'd be minor-only with a beta deadline.)
    

4. One could make the case that menu.api.php should give an example of using the service properly rather than assuming the example is for procedural or test code, but rewriting docs is a separate issue scope. Tagging "needs followup" for that.

Committed to 9.1.x, and cherry-picked to 9.0.x and 8.9.x. Thanks!