Writing a resource file from an AST transformation

I am porting a Java annotation processor over to Groovy as an ASTT for
enhanced goodness. One of the responsibilities of this processor is to
write out an SPI manifest file containing the names of annotated classes.
In Java, I can use Filer#createResource to write this.

The closest I've been able to find in Groovy is
`sourceUnit.AST.unit.config.targetDirectory`. Is this the best option?