Flutter生成JSON序列化模板报错

问题详情：

在执行flutter packages pub run build_runner build自动生成JSON序列化模板的时候出现了以下报错：

Generator cannot target libraries that have not been migrated to null-safety.
package:flutter_test2/model/recommender.dart:6:7
  ╷
6 │ class Recommender {
  │       ^^^^^^^^^^^
  ╵
[INFO] Running build completed, took 1.0s
[INFO] Caching finalized dependency graph...
[INFO] Caching finalized dependency graph completed, took 41ms
[SEVERE] Failed after 1.1s
pub finished with exit code 1

JS通过字符串动态获取对象的值和设置动态键名

需求：

1. 获取（或赋值）json对象其中的某个值，但是这个值的键名是不固定（动态）的
2. 为json对象创建一个不固定键名（动态键名）的值

实现1：

通过动态键名获取对象的值：
object['键名']
设置方式当然就是：
object['键名'] = '值'