ImageElement 构造函数
实现
factory ImageElement({String? src, int? width, int? height}) {
ImageElement e = JS<ImageElement>(
'returns:ImageElement;creates:ImageElement;new:true',
'#.createElement(#)',
document,
"img");
if (src != null) e.src = src;
if (width != null) e.width = width;
if (height != null) e.height = height;
return e;
}