function span(classes, children, height, depth, maxFontSize, style) {
                    this.classes = classes || [];
                    this.children = children || [];
                    this.height = height || 0;
                    this.depth = depth || 0;
                    this.maxFontSize = maxFontSize || 0;
                    this.style = style || {};
                    this.attributes = {};
                }
Provide an `indexOf` function which works in IE8, but defers to native if
possible.