title: Object.assign Babel with external npm dep
body: <p>I have an external NPM dependency that uses Object.assign. I want to use Babel, to assure compatibility with old browsers, either transforming Object.assign via helper or substituting it using another method, but it is not working.</p>
  
  <p>PS: I can use a polyfill at the top of the code, i want to avoid this since it is a library (<a href="https://babeljs.io/docs/plugins/transform-object-assign/" rel="nofollow noreferrer">https://babeljs.io/docs/plugins/transform-object-assign/</a>) </p>
  
  <p>The dependency is : <a href="https://github.com/krux/postscribe" rel="nofollow noreferrer">https://github.com/krux/postscribe</a> 
  via <code>var postscribe = require('postscribe');</code></p>
  
  <p>my .babelrc conf:</p>
  
  <pre><code>{
    "presets": [
       ["env", {
           "targets": {
              "browsers": ["Android 4", "iOS 7"]
           }
       }]
  ],
    "plugins": ["transform-object-assign"]
  }
  </code></pre>
  
  <p>Gulp conf:</p>
  
  <pre><code>gulp.task('adtag', function () {
  // set up the browserify instance on a task basis
  var b = browserify({
      entries: './src/adtag/main.js',
      debug: true
  }).transform(babelify)
  
  return b.bundle()
      .pipe(source('smaatoAdTag.min.js'))
      .pipe(buffer())
      .pipe(sourcemaps.init({
          loadMaps: true
      }))
      // Add transformation tasks to the pipeline here.
      .pipe(uglify())
      .on('error', gutil.log)
      .pipe(sourcemaps.write('./'))
      .pipe(gulp.dest('./dist/adtag/'));
  }); 
  </code></pre>
  
  <p>Is there any way to substitute Object.assign if is in a dependency and not in the main code?</p>