Inspecting ModalBottomSheet

When we call `showModalBottomSheet` to show a bottom sheet, It's a little hard to select a widget that exists in the bottom sheet, the inspector selects a widget behind the sheet and we must try and error multiple times to select a widget inside of bottom sheet

I think this is true for dialogs too

首先使用 showModalBottomSheet 重现该问题，并检查 sheet 或对话框可见时 widget 选择的行为。完成标准是 inspector 能够可靠地选择 bottom sheet 和对话框内的 widget，而不是选择它们后面的 widget。

由索引模型根据 Issue 内容生成。