A simple method that reads a Map and returns a new map formatted as form data for https://pub.dartlang.org/packages/http

mapToFormData.dart

Map<String, String> mapNested(Map<String, dynamic> obj, [Map<String, String> current, String namespace]) {
  Map<String, String> res = current != null ? current : Map<String, String>();

  for (String key in obj.keys) {
    String newKey = namespace != null ? '$namespace[$key]' : key;

    if (obj[key] is Map) {
      mapNested(obj[key], res, key);
    } else if (obj[key] is List) {
      for(int i = 0; i < obj[key].length; i++) {
        mapNested(obj[key][i], res, "$key[$i]");
      }
    } else {
      res[newKey] = "${obj[key]}";
    }
  }

  return res;
}

// Example
print(mapToFormData({
  "foo": "bar",
  "baz": [
    {
      "xyz": 123,
      "wub": 456,
    },
    {
      "xyz": 789,
      "wub": 012,
    },
  ],
  "wat": {
    "rly": 0,
    "ya": 1,
  },
}));
// should print
/*
{
  foo: bar,
  baz[0][xyz]: 123,
  baz[0][wub]: 456,
  baz[1][xyz]: 789,
  baz[1][wub]: 12,
  wat[rly]: 0,
  wat[ya]: 1
}
*/