andrew farquharson
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php

+  /**
+   * 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';
+
--- a/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererBubblingTest.php
    $this->assertEquals('Cache context!Cache tag!Asset!Placeholder!barstoolNested!Cached nested!', trim($output), 'Expected HTML generated.');
    $this->assertEquals(['child.cache_context'], $test_element['#cache']['contexts'], 'Expected cache contexts found.');
    $this->assertEquals(['child:cache_tag'], $test_element['#cache']['tags'], 'Expected cache tags found.');
+    // Test the constants for the #cache keys
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
--- a/core/tests/Drupal/Tests/Core/Render/RendererTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
    $this->renderer->renderInIsolation($build);

    $this->assertEqualsCanonicalizing(['languages:language_interface', 'theme', 'user'], $build['#cache']['contexts']);
+    // Test the constants for the #cache keys