Mention ES2015 module implied strictness is off by default

[abernix]

While it doesn't really cause problems for anyone, the guide should probably mention somewhere that the implied strictness in ES2015 modules, as specified by ES2015 spec, is (currently/intentionally) off by default in Meteor.

As I wrote in meteor/meteor#7050 (comment):

This is done on purpose right now to maintain the highest level of reverse compatibility with pre-1.3 versions. See related issue here: meteor/meteor#6166 and commits meteor/meteor@ae90250 & meteor/babel@823ac57 (which intentionally turned it off) and bugs here meteor/meteor#6072 meteor/meteor#6086 (not to mention all the other stuff that could be broken if this was changed right now).

If developers place "use strict"; at the top of their modules, it will still be enabled, but the impression that it is on automatically in modules is not true. Alternatively, as of 1.3.3, users will be able to enforce their own rule on this (overriding Meteor's default) with .babelrc, thanks to meteor/meteor@49a60f1.

ES2015 Modules should probably (briefly) mention this and possibly Code Style.

[abernix]

And thanks to @AnthonyAstige for originally bringing this up in meteor/meteor#7050.

[filipenevola]

I'm closing this issue because it's too old.

We are going to make many updates to the guide in the next weeks and it's better to focus on issues with recent activity. If you think this issue is still relevant please open a new one.