There is a tiny diff:
targetFontFaceDeclsMap[newFontFaceName] = /**@type{!Element}*/(n);
vs.
targetFontFaceDeclsMap[fontFaceName] = /**@type{!Element}*/(n);

Also the first clause has another additional guard, so the other two commands are not in any case.

Duplicating seems the easiest code, but perhaps there is a better pattern for this?