Issue #2353823 by ultimike: D6->D8 Migration missing variable: cache.

core/modules/migrate_drupal/config/install/migrate.migration.d6_system_performance.yml

@@ -8,10 +8,12 @@ source:
     - preprocess_css
     - preprocess_js
     - cache_lifetime
+    - cache
 process:
   'css/preprocess': preprocess_css
   'js/preprocess': preprocess_js
   'cache/page/max_age': cache_lifetime
+  'cache/page/use_internal': cache
 destination:
   plugin: config
   config_name: system.performance

core/modules/migrate_drupal/src/Tests/Dump/Drupal6SystemPerformance.php

@@ -33,6 +33,10 @@ public function load() {
       'name' => 'cache_lifetime',
       'value' => 'i:0;',
     ))
+    ->values(array(
+      'name' => 'cache',
+      'value' => 'i:1;',
+    ))
     ->execute();
   }
 

core/modules/migrate_drupal/src/Tests/d6/MigrateSystemPerformanceTest.php

@@ -40,6 +40,7 @@ public function testSystemPerformance() {
     $this->assertIdentical($config->get('css.preprocess'), FALSE);
     $this->assertIdentical($config->get('js.preprocess'), FALSE);
     $this->assertIdentical($config->get('cache.page.max_age'), 0);
+    $this->assertIdentical($config->get('cache.page.use_internal'), TRUE);
   }
 
 }