Invalid warning for not declaring 'corejs' option when using 'useBuiltIns': 'false'

Bug Report

• I would like to work on a fix!

Current Behavior When using @babel/preset-env and setting useBuiltIns: 'false' and not declaring a corejs option, the following warning occurs:

WARNING: We noticed you’re using the useBuiltIns option without declaring a core-js version. Currently, we assume version 2.x when no version is passed. Since this default version will likely change in future versions of Babel, we recommend explicitly setting the core-js version you are using via the corejs option.\n" + You should also be sure that the version you pass to the corejs option matches the version specified in your package.json’s dependencies section. If it doesn’t, you need to run one of the following commands: npm install --save core-js@2 npm install --save core-js@3 yarn add core-js@2 yarn add core-js@3

First, I don’t know if this should be considered a bug, because the problem occurs only when setting the useBuiltIns option to the string 'false' and not false. But if you set it accidentaly to a string value I think It should behave the same as when setting it to a boolean one.

Expected behavior/code The warning should not occur.

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

• Filename: babel.config.js

{
  presets: [
    "@babel/env",
    {
      useBuiltIns: "false"
    }
  ]
}

Environment

• Babel version: 7.9.0
• Babel preset-env version: 7.9.0
• Node/npm version: 10.17.0
• OS: Ubuntu 18.04.3 LTS
• Monorepo: [e.g. yes/no/Lerna]
• How you are using Babel: [loader]

Possible Solution Upon inspecting the @babel/preset-env source code, in the file normalize-options.js in the function normalizeCoreJSOption the following check is used

if (useBuiltIns && corejs === undefined) {

Using a more thorough check for useBuiltIns will fix the warning.

[JMarkoski]

@JLHwung Yes, I will submit a PR. Also, the same thing should be done with https://github.com/babel/babel/blob/94447135c75a19da867f2e6a8906ce0a7e8f1ed4/packages/babel-preset-env/src/normalize-options.js#L171

I will add a fix for that in the PR as well. Thanks.

[nicolo-ribaudo]

We should throw an error about "false" not being a valid value instead.