added the ability to export entities via a new 'export' tab on the canonical page

default_content.install

+<?php
+
+/**
+ * @file
+ * Installs default content on the behalf of modules at time of install.
+ */
+
+/**
+ * Implements hook_modules_installed().
+ */
+function default_content_modules_installed($modules) {
+  // @todo Move this to an event once we have HookEvent.
+  foreach ($modules as $module) {
+    if (!\Drupal::isConfigSyncing()) {
+      \Drupal::service('default_content.manager')->importContent($module);
+    }
+  }
+}

default_content.links.task.yml

+default_content.tabs:
+  class: \Drupal\Core\Menu\LocalTaskDefault
+  deriver: \Drupal\default_content\LocalTasks

default_content.module

 <?php
 
-/**
- * @file
- * Installs default content on the behalf of modules at time of install.
- */
 
 /**
- * Implements hook_modules_installed().
+ * Implements hook_entity_type_alter().
+ * creates another node form for exporting
  */
-function default_content_modules_installed($modules) {
-  // @todo Move this to an event once we have HookEvent.
-  foreach ($modules as $module) {
-    if (!\Drupal::isConfigSyncing()) {
-      \Drupal::service('default_content.manager')->importContent($module);
+function __default_content_entity_type_alter(array &$entity_types) {
+  foreach ($entity_types as $entity_type_id => $entity_type) {
+
+    if ($entity_type instanceOf Drupal\Core\Entity\ContentEntityType) {
+      $entity_types[$entity_type_id]
+        ->setFormClass('default_content_export', 'Drupal\default_content\ExportEntityForm');
+      $entity_types[$entity_type_id]
+        ->setLinkTemplate('export-form', "/$entity_type_id/{{$entity_type_id}}/export");
     }
   }
-}
+
+}
\ No newline at end of file

default_content.routing.yml

+#entity.node.export:
+# defaults:
+#    _form: \Drupal\default_content\ExportForm
+#    _title: '\Drupal\defaultcontent\Export::title'
+#  requirements:
+#    _permission: 'access site reports'
+#  options:
+#    _admin_route: true

default_content.services.yml

@@ -2,3 +2,10 @@ services:
   default_content.manager:
     class: Drupal\default_content\DefaultContentManager
     arguments: ['@serializer', '@plugin.manager.rest', '@current_user', '@entity_type.manager', '@entity.repository', '@rest.link_manager', '@event_dispatcher', '@module_handler', '@info_parser']
+    
+ 
+  default_content.route.subscriber:
+    class: Drupal\default_content\RouteSubscriber
+    arguments: [@entity_type.manager]
+    tags:
+     - { name: event_subscriber }

src/ExportForm.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\default_content\ExportForm.
+ */
+
+namespace Drupal\default_content;
+
+use Drupal\Core\Form\FormStateInterface;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * Converts a node to a config entity and shows it on screen
+ * We use the EntityForm here because it is an easy way to get the node from the routing system
+ */
+class ExportForm extends \Drupal\Core\Form\FormBase {
+
+  private $default_content_manager;
+  private $entity;
+  /**
+   * Constructs a ContentEntityForm object.
+   *
+   * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
+   *   The entity manager.
+   */
+  public function __construct($entity_type_manager, $route_match, $default_content_manager) {
+    $params = $route_match->getParameters()->all();
+    list($entity_type_id, $id) = each($params);
+
+    $this->entity = $entity_type_manager->getStorage($entity_type_id)->load($id);
+    $this->defaultContentManager = $default_content_manager;
+  }
+
+  static function create(\Symfony\Component\DependencyInjection\ContainerInterface $container) {
+    return new static(
+      $container->get('entity_type.manager'),
+      $container->get('current_route_match'),
+      $container->get('default_content.manager')
+    );
+  }
+
+  function getFormId() {
+    return 'default_content_export_form';
+  }
+
+  function buildForm(array $form, FormStateInterface $form_state) {
+    $form['output'] = [
+      '#title' => $this->t('Output'),
+      '#type' => 'radios',
+      '#options' => [
+        'download' => $this->t('Download'),
+        'screen' => $this->t('Print to screen')
+      ],
+      '#required' => TRUE,
+    ];
+    $form['relations'] = [
+      '#title' => $this->t('Include relations'),
+      '#type' => 'checkbox',
+      '#states' => [
+        'visible' => [
+          ':input[name=output]' => ['value' => 'screen']
+        ]
+      ]
+    ];
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Export'),
+      '#submit' => [[$this, 'submitForm']]
+    ];
+    if ($output = $form_state->get('exported')) {
+      foreach ($output as $entity_type_id => $entities) {
+        foreach ($entities as $uuid => $entity) {
+          $form['exported'] = [
+            '#title' => $entity_type_id,
+            '#type' => 'textarea',
+            '#description' => $this->t('Put this code in a file, in @path', ['@path' => 'MY_MODULE/content/'.$entity_type_id.'/'.$uuid.'.json']),
+            '#value' => $entity,
+            '#input' => FALSE
+          ];
+        }
+      }
+
+    }
+    return $form;
+  }
+
+  protected function getEditedFieldNames(FormStateInterface $form_state) {
+    $fieldnames = getEditedFieldNames($form_state);
+    $fieldnames[] = 'output';
+    $fieldnames[] = 'relations';
+    return $fieldnames;
+  }
+
+  //this is being called statically, for some reason
+  function submitForm(array &$form, FormStateInterface $form_state) {
+    $params = \Drupal::routeMatch()->getParameters()->all();
+    list($entity_type_id, $id) = each($params);
+
+
+    if ($form_state->getValue('relations')) {
+      $entities = $this->defaultContentManager
+        ->exportContentWithReferences($entity_type_id, $id);
+    }
+    else {
+      $entity = \Drupal::entityTypeManager()->getStorage($entity_type_id)->load($id);
+      $json = $this->defaultContentManager->exportContent($entity_type_id, $id);
+      $entities = [
+        $entity_type_id => [
+          $entity->uuid() => $json
+        ]
+      ];
+    }
+
+
+    if ($form_state->getValue('output') == 'download') {
+      if (!isset($entity)) {
+        drupal_set_message('creating of the zip file is not yet implemented');//@todo
+      }
+      $headers = [
+        'Content-Type' => 'application/octet-stream',
+        'Content-Length' => strlen($content),
+        'Content-Disposition' =>  'attachment; filename='.$entity->uuid().'.json'
+      ];
+
+      $content = $entities[$entity_type_id][$entity->uuid()];
+      $response = new Response($content, 200, $headers);
+      $response->send();
+      exit;
+    }
+    else {
+      $form_state->set('exported', $entities);
+      $form_state->setRebuild(TRUE);
+    }
+  }
+
+
+}

src/LinkManager/RelationLinkManager.php

@@ -44,7 +44,7 @@ class RelationLinkManager extends RestRelationLinkManager {
   /**
    * {@inheritdoc}
    */
-  protected function writeCache() {
+  protected function writeCache($context = []) {
     $data = array();
 
     foreach ($this->entityManager->getDefinitions() as $entity_type_id => $entity_type) {

src/LocalTasks.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\default_content\Plugin\Derivative\LocalTasks.
+ */
+
+namespace Drupal\default_content;
+
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+
+/**
+ * Provides local task definitions one to export each entity type
+ */
+class LocalTasks extends DeriverBase implements ContainerDeriverInterface {
+
+  use StringTranslationTrait;
+
+  var $entityTypeManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct($entity_type_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_type.manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    $this->derivatives = [];
+    foreach ($this->entityTypeManager->getDefinitions() as $entity_type_id => $definition) {
+      if ($definition instanceOf \Drupal\Core\Entity\ContentEntityType) {
+        $this->derivatives["default_content.export.".$entity_type_id] = [
+          'route_name' => "entity.$entity_type_id.export",
+          'weight' => 5,
+          'title' => $this->t('Export'),
+          'base_route' => "entity.$entity_type_id.canonical",
+        ];
+      }
+    }
+    debug($this->derivatives);
+    return $this->derivatives;
+  }
+
+}

src/RouteSubscriber.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\default_content\RouteSubscriber.
+ * Makes a new export route for every contentEntity
+ */
+
+namespace Drupal\default_content;
+
+use Drupal\Core\Routing\RouteSubscriberBase;
+use Symfony\Component\Routing\RouteCollection;
+use Drupal\Core\Routing\RoutingEvents;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Subscriber for one route to export each entity type
+ */
+class RouteSubscriber extends RouteSubscriberBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function alterRoutes(RouteCollection $collection) {
+    foreach (\Drupal::entityTypeManager()->getDefinitions() as $entity_type_id => $definition) {
+      if ($definition instanceOf \Drupal\Core\Entity\ContentEntityType) {
+        $route = new Route($definition->getLinkTemplate('canonical') . '/export');
+        $route->setDefaults([
+          '_form' => '\Drupal\default_content\ExportForm',
+          '_title' => 'Export'
+        ]);
+        $route->setRequirements([
+          '_permission' => 'access site reports',
+        ]);
+        $collection->add('entity.'.$entity_type_id.'.export', $route);
+      }
+    }
+  }
+
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return [
+      RoutingEvents::ALTER => ['onAlterRoutes', 100]
+    ];
+  }
+
+}