[javascript mode] Fix a bug in parsing of arrow function with no args

Closes codemirror#4131

Author: marijnh

mode/javascript/javascript.js

@@ -216,7 +216,7 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       var bracket = brackets.indexOf(ch);
       if (bracket >= 0 && bracket < 3) {
         if (!depth) { ++pos; break; }
-        if (--depth == 0) break;
+        if (--depth == 0) { if (ch == "(") sawSomething = true; break; }
       } else if (bracket >= 3 && bracket < 6) {
         ++depth;
       } else if (wordRE.test(ch)) {