Build and watch ECMAscript 6 using traceur.js and Grunt.
Just letting you know that Webstorm can run this fully integrated now including continuous testing through Karma.
Instructions for setting Webstorm up to use traceur: https://www.youtube.com/watch?v=jbfkcmxLLKY
After following those instructions just point karma.conf.js to wherever you told your traceur file watcher to output the es5 files and include the traceur runtime. Something like this: