Add missing case for ES 2018.
@@ -2763,6 +2763,8 @@ public class Compiler extends AbstractCompiler implements ErrorHandler, SourceFi
return Config.LanguageMode.ECMASCRIPT7;
case ECMASCRIPT_2017:
return Config.LanguageMode.ECMASCRIPT8;
+ case ECMASCRIPT_2018:
+ return Config.LanguageMode.ECMASCRIPT2018;
case ECMASCRIPT_NEXT:
return Config.LanguageMode.ES_NEXT;
default:
