Undefined name 'platformViewRegistry'

### Version of the agora_rtc_engine

6.5.x 6.3.x

### Platforms affected

- [ ] Android
- [ ] iOS
- [ ] macOS
- [ ] Windows
- [x] Web

### Steps to reproduce

https://github.com/AgoraIO-Extensions/Agora-Flutter-SDK/pull/2273

### Expected results

no error

### Actual results

from @arthurbcd

> The ui.platformViewRegistry (currently deprecated) will be removed on the next flutter stable release.
Which throws:
Error: Undefined name 'platformViewRegistry'.
The solution is to use dart:ui_web instead of dart:ui, both available in flutter.

### Code sample

Code sample

```dart
[Paste your code here]
```

### Screenshots or Video

Screenshots / Video demonstration

[Upload media here]

### Logs

Logs

```console
[Paste your logs here]
```

### Flutter Doctor output

Doctor output

```console
[Paste your output here]
```