Add opt-out for eslint-webpack-plugin

[mrmckeb]

This PR adds two new flags, documented under "Advanced Configuration".

• ESLINT_NO_DEV_ERRORS - converts errors to warnings in development (no error overlay).
• DISABLE_ESLINT_PLUGIN - the plugin is completely disabled. This will also bring performance benefits for some users.

It was tested with a mixture of warnings and errors, both flags were tested individually.

Resolves #9887.

[chrisahardie]

Instead of removing the plugin entirely, would it be better just to flip the switch on the failOnError option?

[eek]

Instead of removing the plugin entirely, would it be better just to flip the switch on the failOnError option?

I also agree to not remove the plugin altogether. The plugin also has an option emitWarning that will force to only emit warnings instead of errors, leaving the compile to succeed but still show the warnings.

[ianschmitz]

Not a bad idea! What do you think @mrmckeb?

[silverwind]

I think generally one wants to treat errors as warnings during dev build and to treat warnings as errors during prod build (with an opt-out maybe for that one).

[mrmckeb]

Sorry for the slow response, yes we can make this change. I'll take a look this weekend!

[mrmckeb]

Thanks for the feedback on this everyone, this has been adjusted and now emits warnings only when this flag is set.

[mrmckeb]

Please see this discussion, we'd love to get more input before moving forward with this: #10344

[mrmckeb]

This has been updated to reflect the outcome of #10344.

[xfournet]

Thanks! Would be possible to take into account the DISABLE_ESLINT_PLUGIN variable in https://github.com/facebook/create-react-app/blob/0f6fc2bc71d78f0dcae67f3f08ce98a42fc0a57c/packages/react-scripts/scripts/utils/verifyPackageTree.js#L20 to skip the check of eslint & babel-eslint versions ?

[mrmckeb]

That's a good call @xfournet - @iansu @ianschmitz, what do you think?

[mrmckeb]

@xfournet, please see #10499. We'll aim for this to go out with the next release. Thanks for the suggestion!

[xfournet]

@mrmckeb thanks for taking care of that