Update codebase to use ES6 features supported by Node 4.0

Continuing the discussion started in #6356, it would be awesome to upgrade our codebase to benefit from ES6 features that are available after dropping Node < 4 support.

Below are some of the ideas already mentioned:

@mikesherov:

If we’re going to start using es6, we should consider using https://github.com/mohebifar/lebab if we’re going to upconvert.

@ilyavolodin @platinumazure:

For dropping Node < 4, should we also convert the code to use some of the ES6 features at the same time, as well as modify our .eslintrc file for this repository? Or should we leave it as is and change it as we go?

@mysticatea:

I’d like to use eslint-plugin-node 😇

• node/no-deprecated-api - Disallow deprecated API.
• node/no-missing-require - Disallow require()s for files that don’t exist.
• node/no-unpublished-require - Disallow require()s for files that are not published.
• node/no-unsupported-features - Disallow unsupported ECMAScript features on the specified version.

Especially, node/no-unsupported-features would help us.

[kaicataldo]

Some easier wins:

• Template literals and the prefer-template and template-curly-spacing (configured with “never”? Don’t have a strong preference on this one) rules
• Object shorthand notation and the object-shorthand (configured with “always”) and no-useless-computed-key rules

On a side note, I’d be willing to go through and make the necessary changes for both of these (most likely in batches to make it easier).

[mysticatea]

Keep in mind that Node 4 doesn’t fully implement ES6, so we’re still left with a subset of features we can use.

Sure, it’s the reason I suggested node/no-unsupported-features rule. It would warn it if we use unsupported ES6 features.

It sounds good that we enable those 2 rules to start.