bug#15294: 24.3.50; js2-mode parser is several times slower in lexical-binding mode

[Dmitry Gutov]

If I append -*- lexical-binding: t -*- to the first line of js2-mode.el
and eval the buffer, parsing a file takes several (3 to 4) times longer.

To measure:

0. Install js2-mode, e.g. from GNU ELPA.
1. Save
http://mootools.net/download/get/mootools-core-1.4.5-full-nocompat.js to
some local directory, open it in Emacs.
2. If it opens in some other mode than js2-mode, M-x js2-mode.
3. Don't touch the keys, wait until the file is parsed (the parsing gets
interrupted and rescheduled on input). When redisplay works again, we
can re-parse the file again and measure the time it takes.
4. eval-expression (js2-time (js2-reparse t)), wait until the message
area displays the amount of seconds the process took.

To measure the performance in interpreted mode, open js2-mode.el, M-x
eval-buffer and go through the above scenario (1-4).
To measure the performance in compiled mode, M-x byte-compile-file, then
(load "js2-mode.elc"), then go through the scenario.

My measurements:

lexical-binding |       no |      yes |
Interpreted     |    1 sec | 4.26 sec |
Compiled        | 0.63 sec | 1.76 sec |

--
In GNU Emacs 24.3.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.6.4)
 of 2013-09-06 on axl
Bzr revision: 114159 dmantipov@yandex.ru-20130906164012-6j0s6otwkypj9s1z
Windowing system distributor `The X.Org Foundation', version 11.0.11303000
System Description:     Ubuntu 13.04