Warnings don't work with Rollup's onwarn function

[Pauan]

With version 2.6.1 of rollup-plugin-babel I am getting these errors in the console:

The 'createClass' Babel helper is used more than once in your code. It's strongly recommended that you use the "external-helpers" plugin or the "es2015-rollup" preset. See https://github.com/rollup/rollup-plugin-babel#configuring-babel for more information
The 'classCallCheck' Babel helper is used more than once in your code. It's strongly recommended that you use the "external-helpers" plugin or the "es2015-rollup" preset. See https://github.com/rollup/rollup-plugin-babel#configuring-babel for more information

I have investigated the bundle and I know exactly why it is warning: I am importing a file in node_modules which has been pre-transpiled, so it is using ES6 modules but ES5 syntax, therefore it includes its own internal version of createClass and classCallCheck. From what I understand, this is exactly what package authors are supposed to do.

Ideally rollup-plugin-babel would be smart enough to figure out that this is okay, but in the meantime I would really like to silence these warnings.

So I tried defining an onwarn function in my rollup.config.js:

export default {
  onwarn: function (x) { console.log("test", x); }
};

This works fine for warnings generated by Rollup itself, but does not work for the warnings generated by rollup-plugin-babel (such as the above).