1. override
String debugDescribeChildren(String prefix)

Returns a string describing the current node's descendants. Each line of the subtree in the output should be indented by the prefix argument.

Source 

@override
String debugDescribeChildren(String prefix) {
  return '$prefix \u2558\u2550\u2566\u2550\u2550 text \u2550\u2550\u2550\n'
         '${text.toString("$prefix   \u2551 ")}' // TextSpan includes a newline
         '$prefix   \u255A\u2550\u2550\u2550\u2550\u2550\u2550\u2550\u2550\u2550\u2550\u2550\n'
         '$prefix\n';
}