Update to_source_visitor.dart

There is lost metadata in visitTopLevelVariableDeclaration

Closes https://github.com/dart-lang/sdk/pull/47480
https://github.com/dart-lang/sdk/pull/47480

GitOrigin-RevId: 38e89ffca753998e6547c0d28496a56ce9c9d761
Change-Id: I196923ca0d802bdb6f70f59a6ad5c41679158b5f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217103
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>