There is `KeyCode` class and it already define keycodes. 
https://api.dartlang.org/apidocs/channels/stable/#dart-dom-html.KeyCode@id_UP
Here is example.
https://github.com/sethladd/dart-polymer-dart-examples/blob/master/web/auto_complete/auto_complete.dart#L47
I think we can do this seperate PR.