New rule: no-constant-condition

[feross]

Disallow constant expressions in conditions.

http://eslint.org/docs/rules/no-constant-condition

This rule is overly broad by default, preventing things like while (true) with a break inside, which is really common when writing low-level modules like parsers, etc. which is why it was not enabled before now.

However, there is now a checkLoops option that can be set to false so that only things like this get checked:

if (false) {
  // ...
}

So I propose we enable this now.

[jprichardson]

Ah, sometimes I use this to "trick" standard (if (true) return) with the unreachable code error during development. But overall, this is a good change 👍

[yoshuawuyts]

Aye, works for me

[feross]

This rule will be included in standard v8