Namerdart (#7187)

* Apply Namer to Dart.

- Also refactor idl_gen_dart a bit
  - to use more const and references
  - out parameters should be the last argument

* Add keyword test

* minor fixes

* fix merge

* extra 's'

* move dart keyord into dart dir

* Address comments

* Use $ for escaping keywords
* Outparameters for namespace_map

* Escape dollar in toString

* Escape dollar in toString2

* Use UpperCamelCase for types and variants

* try to fix ToString

* namer Type fixes

* Remove path prefixing in imports

* gen code

Co-authored-by: Casper Neo <cneo@google.com>

dart/test/flat_buffers_test.dart

@@ -14,6 +14,7 @@ import 'package:test_reflective_loader/test_reflective_loader.dart';
 import './monster_test_my_game.example_generated.dart' as example;
 import './monster_test_my_game.example2_generated.dart' as example2;
 import './list_of_enums_generated.dart' as example3;
+import './keyword_test_keyword_test_generated.dart' as keyword_test;
 
 main() {
   defineReflectiveSuite(() {