String encode(Object value, { dynamic toEncodable(object) })

Converts value to a JSON string.

If value contains objects that are not directly encodable to a JSON string (a value that is not a number, boolean, string, null, list or a map with string keys), the toEncodable function is used to convert it to an object that must be directly encodable.

If toEncodable is omitted, it defaults to a function that returns the result of calling .toJson() on the unencodable object.

Source 

String encode(Object value, {toEncodable(object)}) {
  if (toEncodable == null) toEncodable = _toEncodable;
  if (toEncodable == null) return encoder.convert(value);
  return new JsonEncoder(toEncodable).convert(value);
}