[meta] Further modernise our JavaScript coding style rules, ESLint config and code bases

[kzar]

Background

A while back we had a push to use more modern JavaScript features, improve our coding style rules and begin using ESLint (#3692). There's still more work to do however ​as Manish pointed out.

So for the second push I thought I'd create this meta-issue so that we can keep track of the suggestions of how we can improve things as we work through them.

We'll need to check proposed features are supported by the browsers we support (e.g. Chrome 49, Firefox 51, Edge 40 10.0.15063 Creators update, etc), that their use will improve the code base and that we're all agreed about that. Also we'll need to keep our ​JavaScript coding style rules and ​ESLint configuration in sync with what we all agree. (Then of course we'll need to update our codebases as well!) In other words this is quite a bit of work in practice, but I think worthwhile.

Ticket	Status	Resolution	Summary	Component	Owner
#6411	closed	rejected	Allow sparse arrays in eslint-config-eyeo	Automation
#6424	closed	rejected	Replace underscore properties with symbols	Platform
#6433	closed	rejected	Use generators to return lists of things	Platform
#6434	closed	rejected	Define helpers for non-array iterables	Core
#6445	new		[meta] Make use of default function parameters	Unknown
#6447	closed	rejected	Switch to Harmony modules	Core
#6449	closed	rejected	Switch to Harmony modules in adblockpluschrome	Platform
#6482	new		[meta] Use the class keyword in JavaScript code	Unknown
#6564	closed	rejected	Replace desc and extend in lib/coreUtils.js with __proto__	Core
#6581	closed	worksforme	[meta] Clarify the rules for using const vs let in JavaScript code	Unknown
#6737	closed	fixed	Use String.includes instead of String.indexOf where the index is not needed	Core	mjethani
#6959	closed	rejected	Switch to using async/await in core unit tests	Core