void formData=(Map<String, String> args)

Form-data encodes a serialized value into body and sets contentType.

This method will encode args as x-www-form-urlencoded data and set it as the body of this request. args must be a Map<String, String> . The contentType will be set to "application/x-www-form-urlencoded".

Source 

set formData(Map<String, String> args) {
  body = args.keys
      .map((key) => "$key=${Uri.encodeQueryComponent(args[key])}")
      .join("&");
  contentType = new ContentType("application", "x-www-form-urlencoded");
}