Add folding support for plugins and update the main tutorial page

pkg/analyzer_plugin/test/plugin/test_all.dart

Index: pkg/analyzer_plugin/test/plugin/test_all.dart
diff --git a/pkg/analyzer_plugin/test/plugin/test_all.dart b/pkg/analyzer_plugin/test/plugin/test_all.dart
index f0b12832ad3061b76d1f6da24a2d6c17b0e3f640..0182880bf2a34215bd6f5430f67ecd729bec2379 100644
--- a/pkg/analyzer_plugin/test/plugin/test_all.dart
+++ b/pkg/analyzer_plugin/test/plugin/test_all.dart
@@ -7,6 +7,7 @@ import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'assist_mixin_test.dart' as assist_mixin_test;
 import 'completion_mixin_test.dart' as completion_mixin_test;
 import 'fix_mixin_test.dart' as fix_mixin_test;
+import 'folding_mixin_test.dart' as folding_mixin_test;
 import 'highlights_mixin_test.dart' as highlights_mixin_test;
 import 'kythe_mixin_test.dart' as kythe_mixin_test;
 import 'navigation_mixin_test.dart' as navigation_mixin_test;
@@ -18,6 +19,7 @@ main() {
     assist_mixin_test.main();
     completion_mixin_test.main();
     fix_mixin_test.main();
+    folding_mixin_test.main();
     highlights_mixin_test.main();
     kythe_mixin_test.main();
     navigation_mixin_test.main();