ios 怎么沉浸式设置appbar，flutter_tab页面的tab_example不能沉浸式

## Steps to Reproduce
**A small application to reproduce the bug(最小化可复现的demo)**

1. ...
2. ...
3. ...

**Flutter Boost Version**
**Target Platform:**
**Target OS version/browser:**
**Devices:**

## Logs
add your crash log or something else.

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The report refers to the flutter_tab page's tab_example and iOS immersive app-bar behavior; start by locating that example and reproducing it on iOS. The issue provides no concrete steps, environment, logs, or completion criteria, so confirm the expected immersive result before defining the fix and its verification.

Written by the indexing model from the issue text.