babel-preset-es2015-rollup

This is [babel-preset-es2015](http://babeljs.io/docs/plugins/preset-es2015/), minus [modules-commonjs](http://babeljs.io/docs/plugins/transform-es2015-modules-commonjs/), plus [external-helpers](http://babeljs.io/docs/plugins/external-helpers/). Use it wi.