function toHandlers(obj, preserveCaseIfNecessary) {
      const ret = {};
      if (!isObject(obj)) {
          warn(`v-on with no argument expects an object value.`);
          return ret;
      }
      for (const key in obj) {
          ret[preserveCaseIfNecessary && /[A-Z]/.test(key)
              ? `on:${key}`
              : toHandlerKey(key)] = obj[key];
      }
      return ret;
  }
For prefixing keys in v-on="obj" with "on"
@private