Package contains ES2015 code

[Jamie Winder]

I don't know if you consider this to be an issue or not, but the dist code for this package now includes ES2015+ code (as of v3.0.0). For example, const and let. I'm able to transpile this myself of course, but I find it rare to have to do so for a library so I'm not sure if it's intentional or not.

[Xiaoyi Chen]

Thanks for reporting this issue. Looks that babel didn't transform const / let. Will take a look.