Closes the RawSynchronousSocket.
Once closeSync has been called, attempting to call readSync, readIntoSync, writeFromSync, remoteAddress, and remotePort will cause a SocketException to be thrown.
void closeSync();
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dart.dev/stable/2.5.0/dart-io/RawSynchronousSocket/closeSync.html