function effect(fn, scheduler) {
      var watcher = new Watcher(currentInstance, fn, noop, {
          sync: true
      });
      if (scheduler) {
          watcher.update = function () {
              scheduler(function () { return watcher.run(); });
          };
      }
  }
@internal since we are not exposing this in Vue 2, it's used only for
internal testing.