How to use the @ephox/swag.onwarn function in @ephox/swag

gruntUtils.generate(plugins, 'plugin', (name) => {
        return {
          options: {
            treeshake: true,
            format: 'iife',
            onwarn: swag.onwarn,
            plugins: [
              swag.nodeResolve({
                basedir: __dirname,
                prefixes: gruntUtils.prefixes({
                  'tinymce/core': 'lib/globals/tinymce/core'
                }, [
                  [`tinymce/plugins/${name}`, `lib/plugins/${name}/main/ts`]
                ]),
                mappers: [
                  swag.mappers.replaceDir('./lib/core/main/ts/api', './lib/globals/tinymce/core/api'),
                  swag.mappers.invalidDir('./lib/core/main/ts')
                ]
              }),
              swag.remapImports()
            ]
          },

globals: {
      options: {
        configFile: 'src/core/main/json/globals.json',
        outputDir: 'lib/globals',
        templateFile: 'src/core/main/js/GlobalsTemplate.js'
      }
    },

    rollup: Object.assign(
      {
        core: {
          options: {
            treeshake: true,
            format: 'iife',
            onwarn: swag.onwarn,
            plugins: [
              swag.nodeResolve({
                basedir: __dirname,
                prefixes: {
                  'tinymce/core': 'lib/core/main/ts'
                }
              }),
              swag.remapImports()
            ]
          },
          files:[
            {
              src: 'lib/core/main/ts/api/Main.js',
              dest: 'js/tinymce/tinymce.js'
            }
          ]