Add MAC_FF_META to KeyCode constants

[arnaud-secondlayer]

This PR adds the missing declaration of MAC_FF_META key code value to the KeyCode constants.
MAC_Fleur_META (value is 224) is fired by Firefox on macOS when pressing the META key, instead of the common META key value (91).

I kept the same comment as Closure's Keycode enumeration.

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[srujzs]

Looks like it matches the dart:html definition, so LGTM. For formatting, I'd prefer the comment go above the ignore. Thanks!