chore: Upgrade linter and adjust eslint rules

[kumar303]

Closes #992

This upgrades the linter but also adjusts some eslint rules. For some mysterious reason, the linter triggered eslint errors.

[kumar303]

@mstriemer I made some progress but I'm a bit stumped on the new lint errors. It looks like our flowtype plugin is not getting applied somehow. We have specific rules like define-flow-type which don't seem to be taking affect.

[mstriemer]

Hmm, a lot of these seem like eslint isn't understanding the es6 code properly. Not necessarily related to flow.

[kumar303]

The errors I looked at were all for Flow types (not valid javascript). The define-flow-type rule is supposed to suppress those. It says: "Used to suppress no-undef reporting of type identifiers."

[mstriemer]

Looks like the errors are still somehow caused by addons-linter. Leaving it at 0.22.3 passes the build.

[mstriemer]

Looks like the errors start with mozilla/addons-linter@0.20.3...0.20.4 which has dispensary updating eslint to version 4, which is in its dependencies likely by accident. I filed mozilla/dispensary#398 to move that to devDependencies.

Should figure out how to test if that fixes it locally, but either way that change should likely go out.

[kumar303]

Thanks. The eslint 4 dep sounds suspiciously related but still weird.

[mstriemer]

Updating to addons-linter@0.22.8 fixes this for me locally. I think we should still get the Firefox 56 schemas in an addons-linter release before we cut a new web-ext, though.

[coveralls]

Coverage remained the same at 100.0% when pulling 5210eb6 on kumar303:upgrade-linter into d13cc19 on mozilla:master.

[kumar303]

@mstriemer thanks for your help on this! Give me a ping when the schemas are released and we'll cut a release.