dynamic body

An object representing the body of the Response, which will be encoded when used to Request.respond.

This is typically a map or list of maps that will be encoded to JSON. If the body was previously set with a HTTPSerializable object or a list of HTTPSerializable objects, this property will be the already serialized (but not encoded) body.

Source 

dynamic get body => _body;
void body=(initialResponseBody)

Sets the unencoded response body.

This may be any value that can be encoded into an HTTP response body. If this value is a HTTPSerializable or a List of HTTPSerializable, each instance of HTTPSerializable will transformed via its HTTPSerializable.asMap method before being set.

Source 

set body(dynamic initialResponseBody) {
  var serializedBody;
  if (initialResponseBody is HTTPSerializable) {
    serializedBody = initialResponseBody.asMap();
  } else if (initialResponseBody is List) {
    serializedBody = initialResponseBody.map((value) {
      if (value is HTTPSerializable) {
        return value.asMap();
      } else {
        return value;
      }
    }).toList();
  }

  _body = serializedBody ?? initialResponseBody;
}