[Issue #120] fix conflict between the naming of material.SearchBar and ume's SearchBar.

[Atsumi3]

Issue: #120

There was a conflict between the naming of material.SearchBar and ume's SearchBar.
https://api.flutter.dev/flutter/material/SearchBar-class.html

Pull Request Checklist

• I have read the UME contribution document and understand how to contribute, commit the code according to the rules. 我已阅读过 UME 贡献文档，并了解如何进行贡献，按照规则提交了代码
• I have added the necessary comments in code to ensure that other contributors can understand the reason for the change. 我在修改中已经添加了必要的注释，以确保便于其他贡献者理解修改原因
• The code has been formatted by dartfmt before push. 代码在提交前已经经过 dartfmt 进行了格式化
• Change does not involve the adjustment of test cases. Or all existing and new tests are passing. 改动不涉及测试用例调整，或 example 工程与单元测试已经完全跑通

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[Atsumi3]

There was no problem in the develop* branch, so it is a PullRequest to master.

[brave723]

ume SearchBar不能换个名字？

[jixiaoyong]

还有个稍微简略点的改法，使用 hide 隐藏掉第三行的 material.dart 中的SearchBar：

import 'package:flutter/material.dart' hide SearchBar;

[Moshimol]

在哪里文件里呢？