bug#26070: 26.0.50; js-mode slash insertion bug

[Richard Copley <rcopley@gmail.com>]

After inserting a slash before other text in a js-mode buffer,
typing more characters has no effect, until you type C-g to quit.

Recipe to reproduce from 'emacs -Q':
M-x js-mode RET
x   ;; inserts "x"
C-b ;; backward char
/   ;; inserts "/"
x   ;; --- no effect! ---
C-g ;; quit

I couldn't see what code is running, because C-g doesn't enter the
debugger in this situation even when debug-on-quit is true, but I
bisected it to this commit:

commit 862d6438cfa6c6c035033697751f3d002357b024
Author: Tom Tromey <tom@tromey.com>
Date:   Sun Feb 5 11:40:18 2017 -0700

    Recognize JS regexp literals more correctly

In GNU Emacs 26.0.50 (build 6, x86_64-w64-mingw32)
 of 2017-03-12 built on MACHINE
Repository revision: 026c2cbf354fab138a65ad7093f17fbb23edb23c
Windowing system distributor 'Microsoft Corp.', version 10.0.14393
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --prefix=/mingw64 --with-modules
 --enable-locallisppath=/c/emacs/site-lisp 'CFLAGS=-O0 -g -ggdb''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS MODULES

Important settings:
  value of $LANG: ENG
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message puny seq byte-opt subr-x gv
bytecomp byte-compile cl-extra help-mode cconv cl-loaddefs pcase cl-lib
dired dired-loaddefs format-spec rfc822 mml easymenu mml-sec
password-cache epa derived epg epg-config gnus-util rmail rmail-loaddefs
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel dos-w32 ls-lisp disp-table
term/w32-win w32-win w32-vars term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript case-table epa-hook jka-cmpr-hook help
simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs button
faces cus-face macroexp files text-properties overlay sha1 md5 base64
format env code-pages mule custom widget hashtable-print-readable
backquote w32notify dbusbind w32 multi-tty make-network-process emacs)

Memory information:
((conses 16 99056 6862)
 (symbols 56 20091 1)
 (miscs 48 48 87)
 (strings 32 19084 4910)
 (string-bytes 1 584358)
 (vectors 16 14743)
 (vector-slots 8 478865 4018)
 (floats 8 54 62)
 (intervals 56 230 0)
 (buffers 976 11))