[Feature request] [ReactDom] Server rendering with chunked encoding

[DanielHeath]

Feature request

What is the current behavior?
Currently ReactDom.renderToString() must synchronously render a complete component.

This means that react servers which connect to other data sources must wait for those to respond before beginning to send an HTTP response.

What is the desired behavior?
Support server-side use of encoding: chunked by

• Exposing a streaming render API e.g. ReactDom.renderToStream().
• Providing a way for react components to 'block' rendering server-side

What would this improve?
This would enable much lower latency in cases where a server is connecting to multiple backing services to service a request, because content early in the document can be delivered to the browser before all backends have responded.

[DanielHeath]

This is related to #6420 but adds the ability for components to say 'I am not quite ready yet; block rendering' on the server.

[gaearon]

#6420 has landed in React 16.
https://medium.com/@aickin/whats-new-with-server-side-rendering-in-react-16-9b0d78585d67

We have another issue for “blocking rendering” in #1739 so let’s close this one.

[gaearon]

#20970