This is a modification of the tutorial by Chris Sevilleja on scotch.io. The original version only worked if there was one checkbox on a page. This code will work for multiple checkboxes.
A working demo can be found here: https://jsfiddle.net/JesseRWeigel/pL3dzdz0/
Original Tutorial: https://scotch.io/tutorials/create-a-google-polymer-checkbox-using-css3
Original Demo: http://codepen.io/sevilayha/full/jCmgE