[traverseAllChildren] fix out-of-scope var use.

Dear ES6 gods, bring us `let` soon.

This fixes an issue where non-keyed iterables are used as children and the value of `i` would be undefined because its used out of scope. This adds a separately scoped iteration index value and appropriately increments it as iteration continues. Doi.

While I'm in there, make the usage of falsey `nameSoFar` more obvious and more consistent with the existing usage on L115

Test plan: first wrote a test covering this previously untested path. Saw an identical issue as was experienced in development environment. Then ensured test passed after this diff.
Merge pull request #2458 from leebyron/fix-iterable

[traverseAllChildren] fix out-of-scope var use.
Separate React Composite and Class

ReactClass is now composed by ReactCompositeComponent rather than
inherit from it.

The state transition functions currently use ReactInstanceMap to map an
instance back to an internal representation.

I updated some tests to use public APIs. Other unit tests still reach into
internals but now we can find them using ReactInstanceMap.

I will do more cleanup in follow ups. The purpose of this diff is to
preserve semantics and most of the existing code.

This effectively enables support for ES6 classes. All you would need to
expose is ReactClassMixin.