[dart:io] Fuchsia version of SocketBase::AvailableDatagram
[dart:io] Fuchsia version of SocketBase::AvailableDatagram

Fixes https://github.com/flutter/flutter/issues/53590

Change-Id: I6c97dcdc8d9d4ea1679467eb973cc6486bd00184
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141662
Reviewed-by: Zichang Guo <d26af3001798f91912fb1504666bca93c093d553@google.com>
Commit-Queue: Jason Simmons <345008440c885d4763d50f58e545b9839f60f79f@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk