Flutter Sizedbox 是一个 布局组件，用来给 child 添加 tight 约束的，也可以用来添加空白。
width，height是 Sizedbox 的参数
BoxConstraints get _additionalConstraints {
return BoxConstraints.tightFor(width: width, height: height);
}
final BoxConstraints constraints = this.constraints;
if (child != null) {
child!.layout(_additionalConstraints.enforce(constraints),
parentUsesSize: true);
size = child!.size;
} else {
size = _additionalConstraints.enforce(constraints).constrain(Size.zero);
}
enforce 方法根据 _additionalConstraints 返回一个新约束，新约束保证在参数 constraints 的范围之内。
