[react] Allow more return values from stateless components

[LinusU]

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Opening this for some early feedback since I can't figure out how to tackle my last problem.

Basically, my original problem is that I have a stateless component that returns an array (it returns the return value of React.Children.map).

When fixing this I stumbled onto another problem; it's illegal to return undefined from a render-function, or from a stateless component. Yet this is currently allowed for normal components.

I think I got most of it right now, but for some reason I'm getting the following error:

test/tsx.tsx(13,1): error TS2605: JSX element type 'ReactRenderResult' is not a constructor function for JSX elements.
  Type 'null' is not assignable to type 'ElementClass'.

whenever a stateless component is used in JSX:

<StatelessComponent />;

Any help would be much appreciated! 🍻

[dt-bot]

types/react/index.d.ts

to authors (@pspeter3 @vsiao @johnnyreilly @bbenezech @pzavolinsky @digiguru @ericanderson @morcerf @tkrotoff @DovydasNavickas @onigoetz @richseviora @theruther4d AssureSign (account can't be detected) Microsoft (account can't be detected)). Could you review this PR?
👍 or 👎?

[typescript-bot]

@LinusU The Travis CI build failed! Please review the logs for more information. Once you've pushed the fixes, the build will automatically re-run. Thanks!

[vsiao]

I love that we can write these tests.

[vsiao]

As for the SFC error, I think it may actually be a problem to be solved at the compiler level. There's some background in microsoft/TypeScript#5478 about how SFCs work. If @RyanCavanaugh could chime in here that would be best.

[mhegazy]

Thanks for your contribution. This PR has failing CI tests and can not be merged in at the time being. For housekeeping purposes we are closing stale PRs. If you'd still like to merge this code in, please open a new PR that has been merged and rebased with the master branch.