Update deprecated features in ESLint configuration files.

[elas7]

1. experimentalObjectRestSpread option.

From the documentation:

Since ESLint v5, ecmaFeatures: { experimentalObjectRestSpread: true } has been equivalent to ecmaVersion: 2018, and has also emitted a deprecation warning. In ESLint v6, the experimentalObjectRestSpread feature has been removed entirely and has no effect. [...]

To address: If you use the experimentalObjectRestSpread option, you should change your config to contain this instead:

{
  "parserOptions": {
    "ecmaVersion": 2018
  }
}

I removed it and updated ecmaVersion from 8 to 9 to keep the number structure. It is equivalent to 2018 (source).

I tested it with yarn lint, and it passed.

2. globals with boolean values.

From the documentation:

To configure global variables inside of a configuration file, set the globals configuration property to an object containing keys named for each of the global variables you want to use. For each global variable key, set the corresponding value equal to "writable" to allow the variable to be overwritten or "readonly" to disallow overwriting. [...]

For historical reasons, the boolean value false and the string value "readable" are equivalent to "readonly". Similarly, the boolean value true and the string value "writeable" are equivalent to "writable". However, the use of older values is deprecated.

Currently, all globals are set to true. To update them and keep the functionality strictly the same, I could have updated them to "writeable". However, linting works fine with "readonly" too, which seems preferable because it produces a stricter test.

I tested it with yarn lint, and it passed.

I also tested the configurations in scripts/rollup/validate by running yarn build-combined && yarn lint-build, which are the commands that use this configuration in CircleCI. It also passed.

[sizebot]

Comparing: f320ef8...8ce9b42

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	129.98 kB	129.98 kB	=	41.56 kB	41.56 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	134.74 kB	134.74 kB	=	42.96 kB	42.96 kB
facebook-www/ReactDOM-prod.classic.js	=	424.31 kB	424.31 kB	=	78.20 kB	78.20 kB
facebook-www/ReactDOM-prod.modern.js	=	412.87 kB	412.87 kB	=	76.47 kB	76.47 kB
facebook-www/ReactDOMForked-prod.classic.js	=	424.31 kB	424.31 kB	=	78.20 kB	78.20 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 8ce9b42

[gaearon]

thanks!