Why compiler assuming `json['x'] = int?`?
Because the [] operator on Map returns a nullable by spec:
V? operator [](Object? key)The value for the given
key, ornullifkeyis not in the map.
https://api.dart.dev/stable/2.15.1/dart-core/Map/operator_get.html
So if you are asking for a key that is not in your Map you will get a null value back and not an exception.
If you are 100% sure json['x'] will always work and want the application to crash in case this is not the case, you can use json['x']!. Alternative, you need to provide default values or other type of handling in case these values is not in the map.