bug#20466: 25.0.50; REGRESSION in `isearch-mode-map': <backspace> is not translated to DEL

[Drew Adams <drew.adams@oracle.com>]

This regression was apparently introduced in Emacs 24.4.

(define-key isearch-mode-map (kbd "DEL") 
            (lambda () (interactive) (message "@@@@@@@@@@@@@@@@")))

During Isearch, hit the Backspace key.  `DEL' is not used, so the
message is not seen.  `isearch-mode-map' shows that `DEL' is correctly
bound to the above command, but `<backspace>' is bound to
`isearch-delete-char'.  It is not translated to `DEL', as is the case in
Emacs generally (still), but it instead now has its own explicit binding
in `isearch-mode-map'.

Why?  This is an unexpected (and unnecessary?) obstacle for users.
It is an incompatible change, and I see nothing in NEWS about it.
Was it an oversight or intentional?

In Emacs prior to 24.4, the message is shown, and `isearch-mode-map'
shows that `DEL' is bound to the above command and there is no binding
for `<backspace>'.  Because there is no binding for it, it gets
translated to `DEL' (as is true in Emacs generally, even in 24.4+).

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'