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