Object.assign 不是函数

I'm using babel with gulp and create a simple DOM library in ES6. But after running and when i'm going to use it, I got the Object.assign is not a function in chrome console.

这是 gulp 代码

gulp.task('scripts', function() {
    return gulp.src(src + 'js/*.js')
      .pipe(babel())
      .pipe(concat('main.js'))
      .pipe(gulp.dest(dest + 'js'));
});

这是类文件

class DOM {
    constructor( selector ) {
        var elements = document.querySelectorAll(selector);

        this.length = elements.length;

        Object.assign(this, elements);
    }

    ...

}

const dom = selector => new DOM(selector);

我在客户端使用它，例如 dom('#elId');