Don't mutate passed-in props, take 2
Fix potential memory leak when unmounting

Fixes #781.
Spy on purgeID instead of unmountIDFromEnvironment
Merge pull request #640 from spicyj/immutable-props-2

Don't mutate passed-in props, take 2
Merge pull request #786 from spicyj/gh-781

Fix potential memory leak when unmounting
Add --harmony option to jsx.
Remove CallbackRegistry