Explicit import for Platform and kIsWeb (#1898)

Following the ideas in flutter/website#7798 I have changed the imports
needed for Platform and kIsWeb to be explicitly named.

## Pre-launch Checklist

- [ ] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [ ] I signed the [CLA].
- [ ] I read the [Contributors Guide].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

code_sharing/client/lib/main.dart

@@ -1,5 +1,5 @@
 import 'dart:convert';
-import 'dart:io';
+import 'dart:io' show Platform;
 import 'package:http/http.dart' as http;
 import 'package:shared/shared.dart';
 import 'package:flutter/material.dart';