if null expression, analyzer need enhanced type verification

```
class Agent {
late int id;
late String name;

Agent.fromJson(Map json) {
id = json['id'] ?? 0;
name = json['name'] ?? '';
}
}
```

When the previous data type is `dynamic`, analyzer can't check that the type of 0 and 'name' match properly;
`json['name ']` or `0` is finally assigned to `name`. Wouldn't it be better if `json['name']` and `0` were verified with `name` respectively? `json['name']` is `dynamic`, so it will not be verified, but `0` is not, then, the verification of `name` and `0` can not be ignored.

Start with the supplied Agent.fromJson example and reproduce the analyzer's current diagnostics for the ?? expression. No files or tests are named, so first identify the relevant language specification rule and analyzer tests. Done means the intended verification behavior for the non-dynamic fallback is defined and covered by tests.

Written by the indexing model from the issue text.