import 'dart:convert';

    void main() {
        var jsonString = """
          {
            "cats": {
                    "abysinnian": {
                        "origin": "Burma",
                        "behavior": "playful"
                    }
                }
        }
      """;

      var obj = JSON.decode(jsonString);

      print(obj['cats']['abysinnian']['behavior']); // playful
    }

See example on dartpad: https://dartpad.dartlang.org/7d5958cf10e611b36326f27b062108fe