[RFC] Use Babel

[oliviertassinari]

In order to build on top of the same foundation and to save time, I think that we should reuse the existing build steps of the main repository by leveraging Babel. It appears that reproducing the same building pipeline will provide the maximum consistency. For instance

• we would like to remove the requirement from yarn link & yarn build in order to work on the documentation or the storybook.
• we will likely want to keep the same production optimizations: https://github.com/mui-org/material-ui/blob/2ed7b23dc35cf72265065506e406427dbcf06a77/babel.config.js#L35.
• we would like to rely on our browserlist to guarantee browser support, something TypeScript doesn't support Suggestion: target specific runtimes instead of JS versions microsoft/TypeScript#19183
• we don't want to minimize the source in the npm package

[oliviertassinari]

This also includes replacing rollup-plugin-typescript2 in favor of @rollup/plugin-babel.

[dtassone]

This also includes replacing rollup-plugin-typescript2 in favor of @rollup/plugin-babel.

Can we use both?
How are we going to check the types without typescript?

This is my main concern with Babel.
We need compile errors for issues with types.

[oliviertassinari]

How are we going to check the types without typescript?

@dtassone It depends on what you have in mind.

• If you are referring to the CI. The solution is to run the TypeScript linter, for instance.
• If you are referring to get TypeScript fails when developing, we don't want that, it slows down the speed of iteration. We don't block the build because eslint fails. Instead, we look at the warning in the IDE and we fix them in time. Shouldn't it be the same with TypeScript? See Opting out of typescript checks in Next 9.0+ vercel/next.js#7687 for more details.

Notice that we need to do run a TypeScript linter (outside of the build step) no matter what, to run the future TypeScript focused tests. For instance:

• https://github.com/mui-org/material-ui/blob/e0e9b42835c229425e03201bb18628822f11f35b/packages/material-ui-lab/src/useAutocomplete/useAutocomplete.spec.ts
• https://github.com/mui-org/material-ui-pickers/blob/719f860bffd3e3f3fa493d4e5988e1b8e9a89880/lib/src/__tests__/typescript/ThemeDefaultProps.tstest.tsx