anchor.diff:    /// alignment.
     static const _spacesPerTab = 4;
   
  +  /// Creats a [Highlighter] that will return a message associated with [span]
  +  /// when [write] is called.
  +  ///
  +  /// [color] may either be a [String], a [bool], or `null`. If it's a string,
  +  /// it indicates an [ANSI terminal color
  +  /// escape](https://en.wikipedia.org/wiki/ANSI_escape_code#Colors) that should
  +  /// be used to highlight the span's text (for example, `"\u001b[31m"` will
  +  /// color red). If it's `true`, it indicates that the text should be
  +  /// highlighted using the default color. If it's `false` or `null`, it
  +  /// indicates that the text shouldn't be highlighted.
     factory Highlighter(SourceSpan span, {color}) {
       if (color == true) color = colors.RED;
       if (color == false) color = null;
  
positive.code:   ///
    /// This method should only be called once.
    String highlight() {
      //if (_span.length == 0 && _span.context.isEmpty) return "";
  
      _writeSidebar(end: glyph.downEnd);
      _buffer.writeln();
  
  
negative.code:     // those first.
      var lineStart =
          findLineStart(_span.context, _span.text, _span.start.column);
      assert(lineStart != null); // enforced by [new Highlighter]
  
      var context = _span.context;
      if (lineStart > 0) {
  