"Undefined mixin button-size" in checkbox.css

[xszh]

Since I would like to use some of the element-ui components (I love it~) in project, I would like to build it myself to make it compatible with existing code and integrate with less. But during the build process (using postcss-salad), it shew error indicating "Undefined mixin button-size" in checkbox.css.

I think it is raised by missing "@import ./mixins/_buttons.css' at the header of "checkbox.css"

Let me know if i missed something, thanks