Port "Trim flag" Rules action to 8.x

+++ b/src/Plugin/Action/FlagTrim.php
@@ -0,0 +1,62 @@
+ *   },

I think that trailing comma in the annotation breaks the tests.

I remember an issue was filed on Doctrine to allow trailing commas.

+++ b/src/Entity/Flag.php
@@ -553,6 +554,48 @@ class Flag extends ConfigEntityBundleBase implements FlagInterface {
 
   /**
+   * Trim a flag to a certain size.
+   *
+   * @param UserInterface $account
+   *   The user object on behalf the trimming will occur.
+   * @param $cutoff_size
+   *   The number of flaggings allowed. Any flaggings beyond that will be trimmed.
+   * @param $trim_newest
+   *   An optional boolean indicating whether to trim the newest flags.
+   * @param $permissions_check
+   *   (optional) A boolean indicating whether to skip permissions.
+   *   This will trim the flag if $permissions_check is TRUE even if the user
+   *   doesn't have the permission to flag/unflag.
+   */
+  public function trim(UserInterface $account, $cutoff_size, $trim_newest, $permissions_check = FALSE) {

This probably belongs in the service rather than the flag class. Or at least, there's discussion going on at the moment about whether the flag/unflag() methods belong in the service of the flag class, and this should go in the same place.