Returns a Future that completes once all buffered data is accepted by the underlying StreamConsumer.
This method must not be called while an addStream is incomplete.
NOTE: This is not necessarily the same as the data being flushed by the operating system.
Future flush();
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dartlang.org/stable/2.0.0/dart-io/IOSink/flush.html