_getTransitionFunction(opts) {
    return (done) => {
      opts.item.applyCss(opts.styles);
      this._whenTransitionDone(opts.item.element, opts.callback, done);
    };
  }
Return a function which will set CSS styles and call the `done` function
when (if) the transition finishes.
@param {Object} opts Transition object.
@return {function} A function to be called with a `done` function.