Fix regression due to being removed.

Closes #868

mode/rst/rst.js

@@ -18,17 +18,7 @@ CodeMirror.defineMode('rst', function(config, options) {
     }
 
     function hasMode(mode) {
-        if (mode) {
-            var modes = CodeMirror.listModes();
-
-            for (var i in modes) {
-                if (modes[i] == mode) {
-                    return true;
-                }
-            }
-        }
-
-        return false;
+        return mode && CodeMirror.modes.hasOwnProperty(mode);
     }
 
     function getMode(mode) {