<NME> gulpfile.js
<BEF> 'use strict';

let babel = require('gulp-babel');
let es = require('event-stream');
let glob = require('glob');
let gulp = require('gulp');

gulp.task('default', ['src', 'test']);

gulp.task('src', () =>
    gulp.src('src/mixwith.js')
        .pipe(babel())
        .pipe(gulp.dest('.'))
        .pipe(gulp.dest('build')));

gulp.task('test', () =>
    es.merge(
        gulp.src('test/*.js').pipe(babel()),
        gulp.src(['test/mocha.opts']))
    .pipe(gulp.dest('build/test')));

<MSG> Move babel config from .babelrc to gulp

<DFF> @@ -9,7 +9,13 @@ gulp.task('default', ['src', 'test']);
 
 gulp.task('src', () =>
     gulp.src('src/mixwith.js')
-        .pipe(babel())
+        .pipe(babel({
+          'plugins': [ 'transform-es2015-modules-umd' ],
+          'only': [
+            'src/*.js',
+            'test/*.js',
+          ]
+        }))
         .pipe(gulp.dest('.'))
         .pipe(gulp.dest('build')));
 

Move babel config from .babelrc to gulp