Support native arrow functions as stateless components

[zpao]

I tested this with arrow functions in Chrome (just modified the non-transforming basic example).

We could simulate part of the issue with arrow functions in the test (undefined prototype), but not the newable part. So we'll want a proper test when we can switch to node 4 and have some tests that don't transform.

Fixes #4856

[zpao]

cc @sebmarkbage @spicyj

[glenjamin]

Native arrow functions have typeof == function, and can never have a prototype property at all, so using 'prototype' in f is probably are a more reliable approach:

> node
> a = () => 1
[Function]
> a
[Function]
> a.prototype
undefined
> 'prototype' in a
false
>
> typeof a
'function'
> b = function(){}
[Function]
> b.prototype
{}
> 'prototype' in b
true
> delete b.prototype
false
> 'prototype' in b
true
> b.prototype = undefined
undefined
> 'prototype' in b
true

[zpao]

That's a good point. We won't be able to test at all in that case but perhaps that's ok…

[glenjamin]

You could do something like try { eval("() => 1") } catch(ex) {} to detect and test only when arrow functions exist?

I guess that's not as relevant because the testsuite locks itself to v0.10?

[sebmarkbage]

Native fn.bind() also gives you a prototype less function. Should be able to use that to test even with transform and older VMs.

[zpao]

Updated for comments. I didn't say it in a comment but I explicitly used .bind instead of of using an arrow function and letting Babel transform since we don't know what that will do (right now it's not high compliance mode so won't actually call .bind)

[sebmarkbage]

lgtm