ES6 modules tree-shaking

Created by: iamart

Dash.js is written in es6 but by default es5 entry file is used by npm. Since a switch to es6 main file would break backward compatibility of the package, adding following line of code to dash.js' package.json would allow projects using es6 modules with bundlers like webpack or rollup tree-shake more accurately and save ~150kb. "module": "index.js", P.S. I could do a PR but seems like I have to sign a form to do so.