[json_codable] the object passed to fromJson is not necessarily a Map

[cedvdb]

Currently json_codable assumes that a Map is passed to the fromJson constructor when that constructor is defined by the user.

This is not supported:

@JsonCodable()
class A {
  final CustomStringSerialization customStringSerializationField;
}

class CustomStringSerialization {
  final String a;

  CustomStringSerialization(this.a);

  String toJson() => a;

  factory CustomStringSerialization.fromJson(String a) =>
      CustomStringSerialization(a);
}

[dart-github-bot]

Summary: json_codable wrongly assumes fromJson receives a Map. The example shows a fromJson accepting a String, causing a failure.

[jakemac53]

cc @dart-lang/core-package-admins curious for your thoughts on the design here. See the corresponding PR as well.

I am currently leaning in the favorable direction, I don't see any real downside to the approach.