Not really sure what's being asked here; but using Transformers you can rewrite code (this is how dart2js and Polymer work). It won't allow you to change any syntax, however.
You could, for example, write a transformer that detected an @log
annotation and then inject code to log that the method was called. Or you could create a @protected
annotation and inject permission-checking code.
Other than transformers, I don't believe there's anything built-in; but I do suspect they'll add a lot of flexibility in this area.