For exported, generated templates, declare with `final` so `dart2js` knows they will never be reassigned. For non-exported, generated change detector classes, prefix the classname with `_` to mark them as internal. Closes #5009
For exported, generated templates, declare with `final` so `dart2js` knows they will never be reassigned. For non-exported, generated change detector classes, prefix the classname with `_` to mark them as internal. Closes #5009