Provide function to get the current system timezone name

[DartBot]

This issue was originally filed by greg...@gmail.com

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It would be helpful if there was a cross platform way to fetch the iana time zone name for the server where the dart vm is currently running.

On Unix this is stored in /etc/timezone, for example on my computer it is "Pacific/Auckland".

I don't think this is always possible to do this correctly in the browser. Probably best to leave for a library there.

Note: DateTime.now().timeZoneName returns the abbreviated time zone name, which is ambiguous (CST Austrailia, and US), and differs depending on the time of the year. i.e. timeZoneName for me is currently NZDT, but in winter is NZST.

[lrhn]

cc @floitschG.

[lrhn]

Added Library-Core, Area-Library, Triaged labels.

[alanrussian]

The Flutter team I work on has a need for this. We can work around it by creating a plugin, but I would be happy to have this supported directly.