Add utilities to support the outline notification

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 48a3a8b58f5e95c23198b924c91a851b01979f02..94a42256f13fdfac083489a4e7976bf28bd9a65e 100644
--- a/pkg/analyzer_plugin/test/plugin/test_all.dart
+++ b/pkg/analyzer_plugin/test/plugin/test_all.dart
@@ -9,6 +9,7 @@ import 'completion_mixin_test.dart' as completion_mixin_test;
 import 'fix_mixin_test.dart' as fix_mixin_test;
 import 'kythe_mixin_test.dart' as kythe_mixin_test;
 import 'navigation_mixin_test.dart' as navigation_mixin_test;
+import 'outline_mixin_test.dart' as outline_mixin_test;
 import 'plugin_test.dart' as plugin_test;
 
 main() {
@@ -18,6 +19,7 @@ main() {
     fix_mixin_test.main();
     kythe_mixin_test.main();
     navigation_mixin_test.main();
+    outline_mixin_test.main();
     plugin_test.main();
   }, name: 'plugin');
 }