Allow fields to be hidden from eachField iteration.
Switch to "handler" field for TryStatement instead of "handlers".

Esprima is being slow to fix this because of backwards compatibility:
https://code.google.com/p/esprima/issues/detail?id=433

I, however, can use default functions and hidden fields to be more
forgiving, so that Esprima-generated ASTs can still be type-checked and
traversed correctly.