let, etc. I found suggestions that language_in needs to be passed with appropriate ECMAScript version. How can I do that either on the command line or in antbuild.xml?
Also, I'm getting
Unable to locate tools.jar. Expected to find it in /usr/lib/jvm/java-8-openjdk-amd64/lib/tools.jar on the command line but w/o ES6 and newer features the task completes fine.