updateField = function(varId) {
        if (varId) {
          thisField.setValue(varId);
        }
      }
Handle the selection of an item in the variable dropdown menu.
Special case the 'Rename variable...', 'Delete variable...',
and 'New message...' options.
In the rename case, prompt the user for a new name.
@param {!goog.ui.Menu} menu The Menu component clicked.
@param {!goog.ui.MenuItem} menuItem The MenuItem selected within menu.