Flymake and the 'face' property (was: master cd06d17: Fix bug with face-id after restoring from pdump)

[Eli Zaretskii]

João, while debugging the problem uncovered by Hydra, I found out that
Flymake puts its own values of the 'face' property on its faces.  For
example:

  (put 'flymake-error 'face 'flymake-error)

This causes the following trouble:

  (face-id 'flymake-error)
    => flymake-error

This is unexpected, since face-id is supposed to return a numeric ID
of a face, otherwise make-glyph-code will break, for example.

IOW, the 'face' property of a face symbol is reserved and shouldn't be
used by packages in any other way except as explained above.  Can
Flymake please behave in this regard?

Thanks.