bug#26750: 25.2; js-mode missing keywords

[Yuen Ho Wong <wyuenho@gmail.com>]

When in js-mode, some latest Javascript keywords are not
highlighted. After looking at js.el, js--keyword-re seems to be missing
the following:

"as", "await, "async", and "of".

"await" is treated as a "future reserved word" in ES6 and promoted to a
keyword in the latest ES8 spec draft.

"async" is treated as a keyword in the AsyncFunctionExpression rule in
ES8 draft.

"of" is treated as a keyword in the for-of iteration production rule in
ES6.

"as" is treated as a keyword in import * as statements in ES6.

References:
ES6: https://www.ecma-international.org/ecma-262/6.0/
ES8: https://tc39.github.io/ecma262/


In GNU Emacs 25.2.1 (x86_64-apple-darwin16.4.0)
 of 2017-04-24 built on sierra.internal.macports.net
Configured using:
 'configure --prefix=/opt/local --without-ns --without-x --without-dbus
 --without-gconf --without-libotf --without-m17n-flt --without-gpm
 --without-gnutls --with-xml2 --with-modules --infodir
 /opt/local/share/info/emacs 'CFLAGS=-pipe -Os -arch x86_64'
 CPPFLAGS=-I/opt/local/include 'LDFLAGS=-L/opt/local/lib
 -Wl,-headerpad_max_install_names -Wl,-no_pie -arch x86_64''

Configured features:
NOTIFY ACL LIBXML2 ZLIB MODULES

Important settings:
  value of $LC_ALL: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
regexp-opt rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode
easymenu cl-loaddefs pcase cl-lib mail-prsvr mail-utils term/xterm xterm
time-date mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type tabulated-list newcomment elisp-mode lisp-mode prog-mode
register page menu-bar rfn-eshadow timer select mouse jit-lock font-lock
syntax facemenu font-core 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 charscript case-table epa-hook jka-cmpr-hook help
simple abbrev 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
kqueue multi-tty make-network-process emacs)

Memory information:
((conses 16 84806 4043)
 (symbols 48 18787 0)
 (miscs 40 41 113)
 (strings 32 14158 5341)
 (string-bytes 1 408779)
 (vectors 16 9651)
 (vector-slots 8 378732 18240)
 (floats 8 149 126)
 (intervals 56 197 0)
 (buffers 976 19))