[dart mode] Remove 'operator' as keyword

Closes codemirror#5009

Author: marijnh

mode/dart/dart.js

@@ -12,7 +12,7 @@
   "use strict";
 
   var keywords = ("this super static final const abstract class extends external factory " +
-    "implements get native operator set typedef with enum throw rethrow " +
+    "implements get native set typedef with enum throw rethrow " +
     "assert break case continue default in return new deferred async await covariant " +
     "try catch finally do else for if switch while import library export " +
     "part of show hide is as").split(" ");