I used to work on this. Happy to answer questions.
Here is a good article about Closure Compiler optimizations which J2CL uses as its optimizing compiler: https://medium.com/@thomasdeegan/10-optimizations-closure-co...
This website has Closure Compiler compiled by J2CL running in the browser itself!
https://closure-compiler-debugger.appspot.com/j2cl_debugger....