Extended JS environment requirements polyfills list with a promise

[Kazimirkas]

IE11 needs a Promise polyfill for React 16.7. (not only Set and Map polyfills)
If you are using Lazy and Suspense feature, IE throws a null error. In 'readLazyComponentType' function implementation.

#1552

[alexkrolick]

👍thanks

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[aweary]

We should probably explain this a bit more, as I don’t think Promise is strictly required unless you’re using React.lazy with dynamic import (or using Promise directly).

Internally React models it as “thenable”, which could be a regular object with a then method.

cc @gaearon

[Kazimirkas]

@aweary @gaearon Would It be correct to add after this sentence
...consider including a global polyfill in your bundled application, such as core-js or babel-polyfill.
a sentence like:
Also, if you’re using React.lazy (that requires a Promise or dynamic import), don't forget to add a polyfilled version of it.
?

[gaearon]

I reverted for now to avoid confusion (#1554) but ideally we'd add a small note to the lazy documentation instead so that people who try it actually see it.

[gaearon]

@alexkrolick for future reference, "JS requirements" can't change without a major version. If we add requirements midway it looks like a breaking change. We should avoid this kind of confusion.

[alexkrolick]

Definitely need to add a note to the lazy docs. Otherwise technically it is broken without a polyfill. It's also in the IE11 polyfill for CRA: https://github.com/facebook/create-react-app/blob/master/packages/react-app-polyfill/ie11.js

See #1556