Following comment https://github.com/kripken/emscripten/pull/5720#issuecomment-343533660 modern Closure Compiler requires Java 8 (upstream issue https://github.com/google/closure-compiler/issues/2672).
Following comment emscripten-core/emscripten#5720 (comment) modern Closure Compiler requires Java 8 (upstream issue google/closure-compiler#2672).