Clean up asserts in Tree.

Clean up asserts in Tree.

Document the types of arguments to various tree methods instead of
asserting. In particular, these asserts were causing failures in
python2.4, when reading modes from the index resulted in a long rather
than an int. This is harmless, so the asserts were just dropped.

Change-Id: I4a8f8b1527e0fd9571ebdb760e09230f25ac3918