Rollup removes local variables unless used within a module. This plugin makes sure specified local variables are preserved and kept local. This plugin wouldn't be necessary if es2015 modules would be used.
function rawjs(opts) {
  opts = opts || {}
  return {
    transform: (code, id) => {
      var variable = opts[id.split('/').pop()]
      if (!variable) return code

      var keepStr = '/*rollup-keeper-start*/window.tmp=' + variable +
        ';/*rollup-keeper-end*/'
      return code + keepStr
    },
    transformBundle: (code) => {
      for (var file in opts) {
        var r = new RegExp(opts[file] + '\\$\\d+', 'g')
        code = code.replace(r, opts[file])
      }
      var re = /\/\*rollup-keeper-start\*\/.*\/\*rollup-keeper-end\*\//g
      return code.replace(re, '')
    }
  }
}