Reconsider bundled polyfills

[gaearon]

See my comment in #3200 (comment). We should remove any polyfills that are only necessary for browsers that aren't supported out of the box by React 16 anyway. We should also document the most efficient way polyfill for the lowest possible supported browsers by React 16 (AFAIK IE9). This would be a breaking change.

[viankakrisna]

with the recent changes regarding to babel and browserslist, do we still need to include this file? https://github.com/facebookincubator/create-react-app/blob/next/packages/react-scripts/config/polyfills.js

[andriijas]

@Timer im testing the upcoming react-scripts 2.0 release on my app, if i set last 1 chrome versions in browserlist i dont expect the pollyfills to be bundled, just wanted to check if this issue is till under consideration for 2.0 - thanks

[Timer]

We'll probably do away with the fetch and promise polyfill.

[jpeg729]

I would love to see two js bundles being produced, one to be served via <script type="module" src="modern.js"> and one to be served via <script nomodule src="legacy.js">.

Justification

• Pushing large bundles to evergreen browsers is wasteful. If they support type="module" then they need much less polyfilling.
• Firefox will very soon support type="module".
• Safari 10.1 downloads and runs both bundles, but safari 10.1 is dying out and a tiny bit of boilerplate can prevent both bundles from running.
• IE<=11 may download both bundles, but only runs the legacy one. That said, IE<=11 is mostly used on desktop computers with good internet connections, so the dual download isn't a huge issue.
• Mobile browsers that download both bundles are also dying out.

[Timer]

We're dropping IE 11 support with v2. See more instructions in the migration section when released.