visitTree method
Walk the tree Stylesheet. pretty if true emits line breaks, extra
spaces, friendly property values, etc., if false emits compacted output.
Implementation
@override
void visitTree(StyleSheet tree, {bool pretty = false}) {
  _prettyPrint = pretty;
  _buff = StringBuffer();
  _indent = 0;
  _startOfLine = true;
  visitStyleSheet(tree);
}