Allow comments in flagfile

The --flagfile argument to the compiler is a convenient way to package up the many options needed. However, the options can be surprising, so the flag file would strongly benefit from being able to add comments:

• what this option does
• explanation for why it’s used in the project
• a TODO to change the option to another value, link to a bug

[kylecordes]

Typically a JSON configuration format would not accommodate the thing @alexeagle was hoping for, comments intermixed with configuration. As I understand some systems work around this by accepting yet another format which is not JSON per se, but a kind of hybrid JSON it also allows comments.

https://www.npmjs.com/package/comment-json

However, for the sake of moving things along, swapping the whole thing over to a completely different way of specifying configuration sound like a lot more work than just tolerating comment lines in the existing format. I don’t have a “vote” but if I did I would encourage the effort be spent on numerous essential features instead of migrating to a different configuration format.

[kylecordes]

Vaguely related: rollup comes with a note module resolver. That resolver does not yet support the es2015 key, for the same reasons as discussed earlier here: That they are aiming to support things that are at least somewhat official standards. I made an alternative resolver that does understand these files, suitable for use with angular:

https://www.npmjs.com/package/rollup-plugin-node-resolve-angular

This is a feasible approach for rollup or webpack (although the letter does not need it, see @jasonaden comment about the configuration option). It’s not feasible for Closure because Closure does not have (that I know of) any way to write a plug-in resolver.