Investigate/prototype codegen for property setters.

[jakemac53]

Properties should all have a custom setter, which calls set("propertyName", value). Javascript does this on the fly but in dart we probably need to codegen it. See https://github.com/dart-lang/polymer-dart/issues/520 for a similar issue.

This would make it much easier for outsiders to interact with your element, today they would need to manually call set whenever modifying your properties which is not intuitive.