transitionToSlide_(isMovingForward, nextSlide, callback) {
    scrollTo(0, 0);
    let className = 'slideInRight';

    if (!isMovingForward) {
      nextSlide.moveBeforeFirst();
      className = 'slideInLeft';
    }

    if (this.currentSlide_) {
      if (isMovingForward) {
        this.currentSlide_.moveAfterLast();
      }

      this.currentSlide_.hide();
    }

    nextSlide.show();

    if (this.initialised &&
        this.plugins.touch &&
        this.plugins.touch.isEnabled) {
      DOM.once(nextSlide.el, DOM.getAnimationEvent(), () => {
        nextSlide.el.classList.remove(className);
        callback.call(this, nextSlide);
      });

      nextSlide.el.classList.add(className);
    } else {
      callback.call(this, nextSlide);
    }
  }
Transitions to a slide, without doing the scroll animation. If the page is
already initialised and on mobile device, it will do a slide animation.
@param {boolean} isMovingForward Whether we're going forward or backwards.
@param {Slide} nextSlide Next slide.
@param {Function} callback Callback to be called upon finishing. This is a
sync function so it'll happen on run time.
@private