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) {
  StringBuffer result = new StringBuffer();
  result.writeln('$prefix \u2502');
  int lastIndex = _children.length - 1;
  if (lastIndex < 0) {
    result.writeln('$prefix \u2514\u2500table is empty');
  } else {
    for (int y = 0; y < rows; y += 1) {
      for (int x = 0; x < columns; x += 1) {
        final int xy = x + y * columns;
        RenderBox child = _children[xy];
        if (child != null) {
          if (xy < lastIndex) {
            result.write('${child.toStringDeep("$prefix \u251C\u2500child ($x, $y): ", "$prefix \u2502")}');
          } else {
            result.write('${child.toStringDeep("$prefix \u2514\u2500child ($x, $y): ", "$prefix  ")}');
          }
        } else {
          if (xy < lastIndex) {
            result.writeln('$prefix \u251C\u2500child ($x, $y) is null');
          } else {
            result.writeln('$prefix \u2514\u2500child ($x, $y) is null');
          }
        }
      }
    }
  }
  return result.toString();
}