Add test for isValidDescriptor

Fixes #1164.

Test Plan: grunt test
Merge pull request #1394 from spicyj/gh-1164

Add test for isValidDescriptor
Consolidate innerHTML setting logic

See https://github.com/facebook/react/commit/7eb33ef#commitcomment-5447724.
Merge pull request #1184 from spicyj/setInnerHTML

Consolidate innerHTML setting logic