How can I use ES2016 decorators in karma?

I have set up karma with the karma-babel-preprocessor, and es2015 in the presets.

I'm using decorators in my code but I don't know how to configure karma to accept them. I have been trying for a while unsuccesfully.

Adding plugins: ['transform-decorators'] to the babelPreprocessor options doesn't help.

Any help will be appreciated.