Modernize ES5 to ES6 with lebab

[rattrayalex-stripe]

Builds on #604, targeting the v7.0.0 release

Uses https://github.com/lebab/lebab, running each transform in its own commit.

See the script I added in the first commit (and removed in the last) for the process.

The primary benefits are, IMO, var->const, function->=> and {foo: foo}->{foo}.

cc @stripe/api-libraries
cc @paulasjes-stripe @robz-stripe @ob-stripe

I'm putting a lot of faith in the test suite here, and my confidence isn't 100% that this is bug-free. I'm not totally sure how to proceed with confidence, or who should review.

EDIT: looking through these commit-by-commit, a few I'm reverting because I think they're bad/dangerous.

These I'm not sure about:

1. c1934de I think I should just carefully check that there's code coverage for any possible bugs introduced here. I like the change and would like to keep it.
2. 08e4905 Relatedly, I love this change but do think there's some possibility it could introduce some subtle edge-case bugs. This one however is way too big to audit. I think any bugs that the test suite wouldn't have caught would be of the shape of "a user customizes their stripe-node setup in some way that we support but don't test and there was some implicit reliance on this". I'm not sure how large the surface area for this is...

Note that I made a few changes by hand (which I probably should have done in separate commits in retrospect). They were:

1. This needed to stay function instead of arrow functions:
   

   stripe-node/lib/ResourceNamespace.js

   Lines 16 to 20 in d80c8f5

   	module.exports = function(namespace, resources) {
   	return function(stripe) {
   	return new ResourceNamespace(stripe, resources);
   	};
   	};

2. These needed to move to const (lebab moved var to let, then eslint complained that they were not reassigned):
   

   stripe-node/lib/StripeResource.js

   Lines 341 to 343 in d80c8f5

   	const apiVersion = self._stripe.getApiField('version');
   	requestData = data;
   	const headers = self._defaultHeaders(

3. This needed to stay a function:
   

   stripe-node/test/utils.spec.js

   Line 114 in d80c8f5

   constructor: function() {

[rattrayalex-stripe]

cc @irace-stripe FYI – looks like it should mostly affect you in that it translates var to const at the top of the files.

[paulasjes-stripe]

I ran the tests on node v6.0.0 and v8.0.0. Works as expected in v6, broke on autopagination tests in v8. Turns out it's a node error, upgrading to v8.1 fixed the issue.

Perhaps we should exclude v8 in favour of v8.1.0?

[irace-stripe]

cc @irace-stripe FYI – looks like it should mostly affect you in that it translates var to const at the top of the files.

Noted – a change I’m happy to make!

[paulasjes-stripe]

I'd either revert this or change it to initialize: () => {},

[rattrayalex-stripe]

Yeah, I prefer that style too... Doesn't seem lebab supports it, I think. Will try to see if I can find an eslint autofix...

[paulasjes-stripe]

Nit like before, but I prefer populate: (raw) => {

[rattrayalex-stripe]

@paulasjes-stripe in 6f2dcb6 I used arrow syntax for more methods; there weren't many places I could safely do this.

I also tried to migrate from self to this, but tests started failing and I aborted before rabbit-holing too deeply. Would appreciate a follow-on PR (no urgency, can happen post-release) finishing this if you're willing to submit one!

r? @ob-stripe

[rattrayalex-stripe]

Per IRL,

LGTM