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handler Sockets could be closed from multiple threads (finalizers run during isolate shutdown and the event handler thread). This leads to potential racy behavior. (See https://github.com/dart-lang/sdk/issues/45641) TEST=ci Change-Id: I87900117a4194a71433680f68ed9b6dd31977403 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209849 Reviewed-by: Ryan Macnak <rmacnak@google.com> Commit-Queue: Siva Annamalai <asiva@google.com>