operator [] method
通过key
获取下一个JSON 不存在则返回空的JSON
Implementation
JSON operator [](dynamic key) {
JSON? json;
/// 尝试从数组获取 获取到则直接返回
json = tryFromArray(key);
if (json != null) return json;
/// 尝试从字典获取 获取到则直接返回
json = tryFromObject(key);
if (json != null) return json;
/// 尝试根据数组Key获取 获取到则返回
json = tryFromKeyPath(key);
if (json != null) return json;
/// 获取不到返回空的JSON
return JSON();
}