Resolve the new file contents from the runner

I'm writing tests for my codemod and want to use the runner programmatically instead of shelling out. I have two fixture files, a before and an after. I want to dry-run the before file, get the output and compare it to my after file.

However, when running the runner programmatically with dry run, I don't believe there is any way to receive the new file. The runner should probably resolve an object that contains information about how long it took to run and the new content.