[javascript mode] Parse arrow function params as paramaters, not plain patterns

Closes codemirror#4933

Author: marijnh

mode/javascript/javascript.js

@@ -398,7 +398,7 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
   function expressionInner(type, noComma) {
     if (cx.state.fatArrowAt == cx.stream.start) {
       var body = noComma ? arrowBodyNoComma : arrowBody;
-      if (type == "(") return cont(pushcontext, pushlex(")"), commasep(pattern, ")"), poplex, expect("=>"), body, popcontext);
+      if (type == "(") return cont(pushcontext, pushlex(")"), commasep(funarg, ")"), poplex, expect("=>"), body, popcontext);
       else if (type == "variable") return pass(pushcontext, pattern, expect("=>"), body, popcontext);
     }