Add Symbol prop type

[RaitoBezarius]

Hi there,

As it seemed that no one took #4917, here is my pull request for it.

Some rationale behind the choice as Symbols are essentially polyfilled / transpiled:

• If the typeof is Symbol -- native support or Babel plugin, then it is a Symbol.
• If it behaves like a Symbol (according to Mozilla doc) -- polyfill / no native support, then it is a Symbol.

Is there any case that I could have missed? Let me know if I need to work on it to get it accepted!
Thanks!

[koistya]

Shouldn't it be symbol: createSymbolTypeChecker, (without () at the end)?

[RaitoBezarius]

Calling the createSymbolTypeChecker returns the chainable type checker, right?
As you would do something like: React.PropTypes.symbol.isRequired and not React.PropTypes.symbol().isRequired (am I wrong?)

[gaearon]

There is #6377 though. Maybe you should collaborate 😉

[RaitoBezarius]

@gaearon @puradox Should we continue the PR here? We can backport my commits to its PR or I can backport its commits to my PR. Anyway, I'm very interested in improving the test suite!
If we could test it against the most popular polyfill (es6-promise, core-js, Babel transpilation, etc...), that would be awesome.

[puradox]

@RaitoBezarius We have a bit more conversation over on my PR. You should take a look and we can collaborate together on which approach is best. Take a look at my changes and compare it with yours. From what it looks like, you treated Symbol as a separate data type, whereas I treated it as a primitive.

[RaitoBezarius]

@gaearon @puradox Let's mark it as closed and continue over your PR, if it is okay.