code: function GetSourceLine(message) {
    var script = %MessageGetScript(message);
    var start_position = %MessageGetStartPosition(message);
    var location = script.locationFromPosition(start_position, true);
    if (location == null) return "";
    return location.sourceText;
  }
docs: Returns the source code line containing the given source position, or the empty string if the position is invalid.