diff --git a/core/modules/block/lib/Drupal/block/BlockBase.php b/core/modules/block/lib/Drupal/block/BlockBase.php
index 99150cf..2dd5cde 100644
--- a/core/modules/block/lib/Drupal/block/BlockBase.php
+++ b/core/modules/block/lib/Drupal/block/BlockBase.php
@@ -161,6 +161,18 @@ public function buildConfigurationForm(array $form, array &$form_state) {
         ),
       ),
     );
+    if (count($this->getRequiredCacheContexts()) > 0) {
+      // Remove the required cache contexts from the list of contexts a user can
+      // choose to modify by: they must always be applied.
+      $context_labels = array();
+      foreach ($this->getRequiredCacheContexts() as $context) {
+        $context_labels[] = $form['cache']['contexts']['#options'][$context];
+        unset($form['cache']['contexts']['#options'][$context]);
+      }
+      $required_context_list = implode(', ', $context_labels);
+      $form['cache']['contexts']['#description'] .= ' ' . t('This block is <em>always</em> varied by the following contexts: %required-context-list.', array('%required-context-list' => $required_context_list));
+    }
+
     // Add plugin-specific settings for this block type.
     $form += $this->blockForm($form, $form_state);
     return $form;
@@ -242,10 +254,22 @@ public function getMachineNameSuggestion() {
   }
 
   /**
+   * Returns the cache contexts required for this block.
+   *
+   * @return array
+   *   The required cache contexts IDs.
+   */
+  protected function getRequiredCacheContexts() {
+    return array();
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function getCacheKeys() {
-    return $this->configuration['cache']['contexts'];
+    // Return the required cache contexts, merged with the user-configured cache
+    // contexts, if any.
+    return array_merge($this->getRequiredCacheContexts(), $this->configuration['cache']['contexts']);
   }
 
   /**
diff --git a/core/modules/book/lib/Drupal/book/Plugin/Block/BookNavigationBlock.php b/core/modules/book/lib/Drupal/book/Plugin/Block/BookNavigationBlock.php
index 333a255..6a9cfff 100644
--- a/core/modules/book/lib/Drupal/book/Plugin/Block/BookNavigationBlock.php
+++ b/core/modules/book/lib/Drupal/book/Plugin/Block/BookNavigationBlock.php
@@ -155,40 +155,10 @@ public function build() {
   /**
    * {@inheritdoc}
    */
-  public function buildConfigurationForm(array $form, array &$form_state) {
-    $form = parent::buildConfigurationForm($form, $form_state);
-
-    // Remove the required cache contexts from the list of contexts a user can
-    // choose to modify by: they must always be applied.
-    $context_labels = array();
-    foreach ($this->getRequiredCacheContexts() as $context) {
-      $context_labels[] = $form['cache']['contexts']['#options'][$context];
-      unset($form['cache']['contexts']['#options'][$context]);
-    }
-    $required_context_list = implode(', ', $context_labels);
-    $form['cache']['contexts']['#description'] .= ' ' . t('This block is <em>always</em> varied by the following contexts: %required-context-list.', array('%required-context-list' => $required_context_list));
-
-    return $form;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getCacheKeys() {
-    // Return the required cache contexts, merged with the user-configured cache
-    // contexts, if any.
-    return array_merge($this->getRequiredCacheContexts(), parent::getCacheKeys());
-  }
-
-  /**
-   * Returns the cache contexts required for this block.
-   *
-   * Two cache contexts are required: cache by URL and by user's roles.
-   *
-   * @return array
-   *   The required cache contexts IDs.
-   */
   protected function getRequiredCacheContexts() {
+    // The "Book navigation" block must be cached per URL and per role: the
+    // "active" menu link may differ per URL and different roles may have access
+    // to different menu links.
     return array('cache_context.url', 'cache_context.user.roles');
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemHelpBlock.php b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemHelpBlock.php
index 89a859d..c594b98 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemHelpBlock.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemHelpBlock.php
@@ -120,23 +120,21 @@ public function build() {
   /**
    * {@inheritdoc}
    */
-  public function buildConfigurationForm(array $form, array &$form_state) {
-    $form = parent::buildConfigurationForm($form, $form_state);
-
-    // The help block is never cacheable, because it is path-specific.
-    $form['cache']['#disabled'] = TRUE;
-    $form['cache']['#description'] = t('This block is never cacheable, it is not configurable.');
-    $form['cache']['max_age']['#value'] = 0;
-
-    return $form;
+  public function defaultConfiguration() {
+    // Modify the default max age for the System Help block: help text is static
+    // for a given URL, except when a module is updated, in which case
+    // update.php must be run, which clears all caches. Thus it's safe to cache
+    // the output for this block forever on a per-URL basis.
+    return array('cache' => array('max_age' => \Drupal\Core\Cache\Cache::PERMANENT));
   }
 
   /**
    * {@inheritdoc}
    */
-  public function isCacheable() {
-    // The help block is never cacheable, because it is path-specific.
-    return FALSE;
+  protected function getRequiredCacheContexts() {
+    // The "System Help" block must be cached per URL: help is defined for a
+    // given path, and does not come with any access restrictions.
+    return array('cache_context.url');
   }
 
 }
diff --git a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemMenuBlock.php b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemMenuBlock.php
index c3b5463..b6d1316 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemMenuBlock.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemMenuBlock.php
@@ -48,25 +48,6 @@ public function defaultConfiguration() {
   /**
    * {@inheritdoc}
    */
-  public function buildConfigurationForm(array $form, array &$form_state) {
-    $form = parent::buildConfigurationForm($form, $form_state);
-
-    // Remove the required cache contexts from the list of contexts a user can
-    // choose to modify by: they must always be applied.
-    $context_labels = array();
-    foreach ($this->getRequiredCacheContexts() as $context) {
-      $context_labels[] = $form['cache']['contexts']['#options'][$context];
-      unset($form['cache']['contexts']['#options'][$context]);
-    }
-    $required_context_list = implode(', ', $context_labels);
-    $form['cache']['contexts']['#description'] .= ' ' . t('This block is <em>always</em> varied by the following contexts: %required-context-list.', array('%required-context-list' => $required_context_list));
-
-    return $form;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getCacheKeys() {
     // Return the required cache contexts, merged with the user-configured cache
     // contexts, if any.
@@ -86,14 +67,12 @@ public function getCacheTags() {
   }
 
   /**
-   * Returns the cache contexts required for this block.
-   *
-   * Two cache contexts are required: cache by URL and by user's roles.
-   *
-   * @return array
-   *   The required cache contexts IDs.
+   * {@inheritdoc}
    */
   protected function getRequiredCacheContexts() {
+    // Menu blocks must be cached per URL and per role: the "active" menu link
+    // may differ per URL and different roles may have access to different menu
+    // links.
     return array('cache_context.url', 'cache_context.user.roles');
   }