writeAll method
override
 Iterates over the given objects and writes them in sequence.
If separator is provided, a write with the separator is performed between any two elements of objects.
This operation is non-blocking. See flush or done for how to get any errors generated by this call.
Implementation
void writeAll(Iterable objects, [String separator = ""]);
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    https://api.dart.dev/stable/2.13.0/dart-io/IOSink/writeAll.html