To allow full ES6 usage it may be worth it to switch to the harmony branch of uglifyjs. This would also fix the issue of uglifyjs throwing error with arrow functions.
ERROR in options.js from UglifyJs
Unexpected token: operator (>) [options.js:6454,19]
ERROR in contentscript.js from UglifyJs
Unexpected token: operator (>) [contentscript.js:6454,19]
ERROR in helpers.js from UglifyJs
Unexpected token: operator (>) [helpers.js:6454,19]
To allow full ES6 usage it may be worth it to switch to the harmony branch of uglifyjs. This would also fix the issue of uglifyjs throwing error with arrow functions.