tools: enable additional lint rules

[Trott]

Enable additional likely-uncontroversial lint rules:

• comma-dangle set to prohibit dangling commas on objects and arrays
  that are defined on a single line. Multi-line definitions can use or
  omit a trailing comma.
• no-unused-labels Prohibits defining a label that is not used.
• no-path-concat Prohibits string-concatenation using __dirname and
  __filename. Use path.join(), path.resolve(), or template strings
  instead.
• no-new-symbol disallow use of new operator with Symbol object.
  Violating this rule would result in a TypeError at runtime.

[targos]

LGTM.
CI: https://ci.nodejs.org/job/node-test-pull-request/1714/

[cjihrig]

LGTM. New CI: https://ci.nodejs.org/job/node-test-pull-request/1725/

[rvagg]

I had to look up comma-dangle to see what the implications are, for anyone else interested: http://eslint.org/docs/rules/comma-dangle

The following patterns are considered problems when configured "only-multiline":

/*eslint comma-dangle: [1, "only-multiline"]*/

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

But things like this are still OK:

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,
    2];

I can't find the previous discussion about dangling commas, if you know where it was would you mind @-mentioning anyone that objected to disallowing dangling commas (my recollection of that discussion) to give them a chance to weigh in here @Trott?

LGTM

[Trott]

@rvagg Sure! Although the objection was to the default comma-dangle behavior which flagged about 80 or so lines of our current code base. This far more lenient version flags zero lines of our current code base.

Should be non-controversial, I think, but just in case, as requested by @rvagg, @-mentioning previous objectors: @Fishrock123 @bnoordhuis

Previous discussion is here: #5091

[Fishrock123]

I'm ok with only-multiline.

[Trott]

Landed in 9534f6d