Add support for accepting "ES9"/"ES2018" as a language mode but don't produce it yet.
Once the transpiler and library are updated to accept the mode, we can roll out actually producing it.
@@ -329,6 +329,9 @@ public final class FeatureSet implements Serializable {
return ES7;
case "es8":
return ES8;
+ case "es2018":
+ case "es9":
+ return ES2018;
case "es_next":
return ES_NEXT;
case "ts":
