Tidies whitespace and adds extra computed property test to validate SC.ObjectController changing content to null.
Removes errors that will appear in development from production builds.
Improves the performance and code structure of SC.TreeItemObserver and SC.TreeController. Previously, the tree observer process was extremely inefficient. For one, the tree controller constantly destroyed and recreated item observer instances whenever its content changed. For another, the item observer observed all properties of its items so any property change (even those not associated with tree item content) would cause the item observer to check for changes to the item’s expanded state or children.

For example, in a current app with a one level deep tree of tree branches, the old code created 25 tree item observers and destroyed 21 for a total of 4 actual item observers. The new code simply creates 4 actual item observers one time!