fix(babel): set useBuiltins to false during MODERN_BUILD

[LinusBorg]

closes #1727

when building in modern mode, polyfills still get included by babel-preset-env even though we set targets: { esmodules: true } here and skip applying the default polyfills here

Appereantly babel-preset-env still respects the browserslist field from package.json when useBuiltIns is 'usage' or 'entry'.

This PR sets useBuiltIns: false` when a modern build is running.

I'm think pondering wether this could have any side-effects?

[LinusBorg]

So I've been thinking .... When set useBuiltins: false, babel-preset-env will not add any polyfills automatically anymore, even though the user might expect it since he maybe even set useBuiltIns explicitly.

Right now, I don't see a problem with that because preset-env will only polyfill stuff that's currently standard, and all browsers that support type="module" support the current standard 100%, right?

But if at some point in the future one of those browsers would lag behind on a feature that becomes standard, the "modern" bundle would not receive that polyfill even though the browser might have been configured correctly in browserslist to get it.

Am I thinking correctly here?

If so - is there another way to implement this? Can we somehow reset the browser targets which are defined in package.json's browserslist field while we are building MODERN?

[LinusBorg]

Ok so I think I got it now. from preset-env docs:

You may also target browsers supporting ES Modules (https://www.ecma-international.org/ecma-262/6.0/#sec-modules). When specifying this option, the browsers field will be ignored.

This makes it sound as if browserslist will be ignored, but it seems that it only ignores browsers defined in the plugin options. This may even be a bug?

Edit: Seems to be a bug indeed, or at least in discussion.

Anyway, instead of setting useBuiltins: false and thereby deactivating any and all polyfills, we can use ignoreBrowserslistConfig to skip package.browserslist.

So if in the future one of the browsers supporting esmodules may require a polyfill to run then-current standard JS, useBuiltins will add that polyfill.

Seems much better to me.

Thoughts @yyx990803 ?

[LinusBorg]

This is ready as far as I'm concerned. I think I weighted all the options.

[SoftwareDreamer]

I think this behavior is interesting for performance-minded people, who want to be sure that modern is really a "minimal" build. Therefor: will this be mentioned in the docs?

[LinusBorg]

will this be mentioned in the docs?

We are always happy to accept PRs :) So please go ahead and make a suggestion what you want to be documented better:

https://cli.vuejs.org/guide/browser-compatibility.html#modern-mode