add code from patch #6

Closes #3144600

core/lib/Drupal/Core/Cache/Cache.php

@@ -16,6 +16,21 @@ class Cache {
    */
   const PERMANENT = CacheBackendInterface::CACHE_PERMANENT;
 
+  /**
+   * Use this inside a #cache array to add cache contexts.
+   */
+  const CONTEXTS = 'contexts';
+
+  /**
+   * Use this inside a #cache array to specify cache max age.
+   */
+  const MAX_AGE = 'max-age';
+
+  /**
+   * Use this inside a #cache array to add cache tags.
+   */
+  const TAGS = 'tags';
+
   /**
    * Merges lists of cache contexts and removes duplicates.
    *

[Kristiaan Van den Eynde]

What I am missing here is the other possible entry for #cache: keys

[Kristiaan Van den Eynde]

What is "a #cache array"? Maybe we should phrase it as "the #cache key of a render array"? It's replacing one drupalism with another but at least it points readers in the right direction.