Allow React lifecycle methods to return mixed

[jamiebuilds]

Resolves #1803

This will allow people to write stuff like:

class Component extends React.Component {
  async componentDidMount() {
    // ...
  }
}

[calebmer]

Thank you!

[facebook-github-bot]

@calebmer has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[capaj]

@thejameskyle wouldn't it be better as void | Promise<void> instead of mixed?

IMHO it could be confusing for new react users if they see returns from these methods. We should encourage users not to return anything. Flow should warn about returns IMO.

[jamiebuilds]

It doesn't really seem like the responsibility of Flow to enforce something that does not make a difference at runtime