ES6: Clarify difference between --es_staging vs --harmony flags

[fhemberger]

Quoting @williamkapke #674 (comment)

I guess there could be more clarification on the difference of the --es_staging vs --harmony flags. From the existing page, I concluded they were the aliases of each other. I guess that isn't true:
See: compat-table/node-compat-table#1 (comment)

I've never used those flags myself, can someone explain the difference?
/cc @nodejs/collaborators

[bnoordhuis]

In v4 and v5, they're synonyms for beta-level features: considered complete but not battle-tested yet; can still have bugs.

In v6, --es_staging is --harmony plus a few alpha-level features; currently --harmony_regexp_lookbehind and --harmony_tailcalls but that can change at any time.

[ChALkeR]

Note that --es-staging is labeled as «for internal use only» and exposes more unstable features, while the change that got landed promotes --es-staging over --harmony.

  --es_staging (enable test-worthy harmony features (for internal use only))
  --harmony (enable all completed harmony features)

[fhemberger]

@ChALkeR Good catch, the "for internal use" remark was not present in Node.js 4.3.1. So we should rather document the --harmony flag only and leave out --es_staging?

[ChALkeR]

@fhemberger In Node.js 4.3.1 those two were equivalent.

So we should rather document the --harmony flag only and leave out --es_staging?

/cc @nodejs/documentation

[fhemberger]

@ChALkeR Yes, but in Node.js 6.0 they are not and --es_staging is flagged "for internal use". But in all versions --harmony does the same, so we should use this flag in our documentation and omit the internal flag --es_staging.

[ChALkeR]

But in all versions --harmony does the same,

… except for 0.10/0.12.

[lpinca]

so we should use this flag in our documentation and omit the internal flag --es_staging.

I agree.