[react] Drop support for React v15.x and take advantage of v16.0.0 once released

[reiwa]

Failed prop type: Invalid prop `children` of type `array` supplied to `MuiThemeProvider`, expected a single ReactElement.

In react@16 we can return an array with the render method.
In this way, should not "MuiThemeProvider" allow arrays?

render(
    <MuiThemeProvider theme={theme}>
      <LeftMenu />
      <Content />
      <Snackbar />
    </MuiThemeProvider>,
  document.querySelector('div')
)

Versions

• Material-UI: 1.0.0-beta.2
• React: 16.0.0-beta.3
• Browser: Safari

[oliviertassinari]

@uufish I think that we should wait for officially dropping react@15.x before updating the propTypes and possibly some of our component implementations.

Check list:

• Update the propTypes
• Update the render methods
• Remove Portal dead code
• Upgrade enzyme

[isitrita]

@oliviertassinari React 16 have already been released, will you remove status onHold from this issue?

[oliviertassinari]

@isitrita https://twitter.com/MaterialUI/status/913019335995568128

[istarkov]

One of react-jss deps contains a library theming with non peer react 15 dependency, seems like it caused a bug in some of our projects

[oliviertassinari]

@istarkov This one was for you #8408 ❤️ . Anyway, this PR #8121 might help as Material-UI runtime code do not rely on theming. But if you are saying that it fails at the installation time, you might be able to use this trick: reactjs/react-transition-group#151 (comment)

[oliviertassinari]

I have found one limitation with using the new array children capability of React@16. Preact do not support it (preactjs/preact-compat#432).
So we have a call to make, does supporting preact worth the extra divs in the DOM for React users? #7721

[eyn]

There are quite a few other libraries in the eco system that don't support react 15 yet (zeit/next.js) for one so I'd vote to keep react 15 support for a while. It might also help lessen the curve of upgrading to 1.0 from 0.1x

[leMaik]

@eyn zeit/next.js will support react 16 soon: vercel/next.js#2997 (comment) 🎉

[sakulstra]

fyi: next 4 supports react 16 (and dropped 15 support)