Why does object splatting generate an unneeded function?

[boris-petrov]

From here:

var currentUser, user,
  _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; };

CoffeeScript 2 generates ES2015 code and as I understand Object.assign is standardized in ES2015. So why the unnecessary function?

[lydell]

@boris-petrov See #4674.