SpiderMonkey ESR31 upgrade

[Yves]

Now that the ESR24 upgrade is done (#1886), I've started working on the ESR31 upgrade. You can test the current WIP version on my ​github repository (ESR31 branch). Read the readme there for additional information.

Depends on

#2669

[Niek]

I thought SpiderMonkey 3.1 will be released in July 2014?

"The next release will be SpiderMonkey 31, around July 2014." ​https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey

[Erik Johansson]

The better to start on the upgrade now rather than wait until after it's been released ;)

[Niek]

But in that case you take the risk that you need to change drastically if the guys @Mozilla do so.

[Yves]

There are two big advantages when starting with the upgrade now.

1. If we find problems that need to be fixed in SpiderMonkey, we can still get these changes in before the next release. The Firefox (and SpiderMonkey) development starts in the mozilla central repository where big features get integrated. After that it moves to the "Aurora" stage where usually only important bugfixes get applied. This transition will be around the 28th of April, so it's good to start now.
2. When updating around once a week, it's easier to figure out the reason if a change breaks something. You can check last week's commit logs or ask people what happened last week. That's harder if you have to search in a period of multiple months.

Sometimes there are bugs and it can take a while to figure out it's not a problem in our code, but the advantages outweight this problem.