With the edition of ECMA-262, we now have a new primative type called
Symbol. This primative type should be added to the PropTypes, as users
will eventually be using Symbols in their props for describing unique
and immutable data, such as identifiers.

I believe that I covered all the typical use cases for Symbols.
If you think that I missed one, feel free to contribute.

Most ES6 polyfills will add support by implementing `Symbol` as a
function. This causes `typeof` to return `function` rather than `symbol`
for polyfilled clients.

1) If it is a native Symbol
2) If it is spec-compliant
3) Try to match non-spec compliant polyfill by a instanceof check on Symbol if it exists in the global namespace

…elements

Improve symbol check for (local) polyfills

…s and trailing comma

Fix code-style issue with React guidelines

Let's get more specific with our tests. Symbol has to be a function if
it's a polyfill. There are no object polyfills out there. (If there is,
ping me and I'll add support)

Instead of checking if Symbol is undefined, let's check if it's a
function instead since instanceof is depending that Symbol is already a
function.

There seems to be a bug with medikoo/es6-symbol where the global state
affects the implementation of the polyfill. I found this by running the
individual test file alone then running all the tests using `grunt
test`. I found that it passes when ran alone and failed when ran
collectively.

I did not find this buggy behavior with zloirock/core-js's
implementation of a polyfill for Symbol. Thus, removing I will keep the
more popular polyfill (core-js) and remove the buggy polyfill
(es6-symbol).

If you are reading this and can think of a confounding factor that is
causing this bug, please let me know and we can try to work together to
add support for medikoo/es6-symbol.

Remove unused ES6 Symbol dependency