Encoding class does not support UTF- encodings

[munificent]

File.writeAsString() accepts an encoding. However, there are no instances of the Encoding class that map to any of the UTF- types except for UTF-8: no UTF-16, UTF-32, etc.

[iposva-google]

Added Area-IO, Triaged labels.

[andersjohnsen]

Yes, adding other UTF encoding is on the library roadmap.

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Removed Area-IO label.
Added Area-Library label.

[floitschG]

Set owner to @floitschG.
Added Accepted label.

[lrhn]

Added Library-Convert label.

[lrhn]

Removed Type-Defect label.
Added Type-Enhancement label.